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Wikipedia

Radio

Radio is the technology of signaling and communicating using radio waves.[1][2][3] Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 3,000 gigahertz (GHz). They are generated by an electronic device called a transmitter connected to an antenna which radiates the waves, and received by another antenna connected to a radio receiver. Radio is widely used in modern technology, in radio communication, radar, radio navigation, remote control, remote sensing, and other applications.

A variety of radio antennas on Sandia Peak near Albuquerque, New Mexico, US

In radio communication, used in radio and television broadcasting, cell phones, two-way radios, wireless networking, and satellite communication, among numerous other uses, radio waves are used to carry information across space from a transmitter to a receiver, by modulating the radio signal (impressing an information signal on the radio wave by varying some aspect of the wave) in the transmitter. In radar, used to locate and track objects like aircraft, ships, spacecraft and missiles, a beam of radio waves emitted by a radar transmitter reflects off the target object, and the reflected waves reveal the object's location. In radio navigation systems such as GPS and VOR, a mobile navigation instrument receives radio signals from navigational radio beacons whose position is known, and by precisely measuring the arrival time of the radio waves the receiver can calculate its position on Earth. In wireless radio remote control devices like drones, garage door openers, and keyless entry systems, radio signals transmitted from a controller device control the actions of a remote device.

The noun radio is also used to mean a broadcast radio receiver.

The existence of radio waves was first proven by German physicist Heinrich Hertz on November 11, 1886.[4] In the mid 1890s, building on techniques physicists were using to study electromagnetic waves, Guglielmo Marconi developed the first apparatus for long-distance radio communication,[5] sending a wireless Morse Code message to a recipient over a kilometer away in 1895,[6] and the first transatlantic signal on December 12, 1901.[7] Karl Ferdinand Braun invented the phased array antenna in 1905.[8] The first commercial radio broadcast was transmitted on November 2, 1920, when the live returns of the Harding-Cox presidential election were broadcast by Westinghouse Electric and Manufacturing Company in Pittsburgh, under the call sign KDKA.[9]

The emission of radio waves is regulated by law, coordinated by the International Telecommunication Union (ITU), which allocates frequency bands in the radio spectrum for various uses.

Etymology edit

The word "radio" is derived from the Latin word "radius", meaning "spoke of a wheel, beam of light, ray". It was first applied to communications in 1881 when, at the suggestion of French scientist Ernest Mercadier [fr], Alexander Graham Bell adopted "radiophone" (meaning "radiated sound") as an alternate name for his photophone optical transmission system.[10][11]

Following Heinrich Hertz's discovery of the existence of radio waves in 1886, the term "Hertzian waves" was initially used for this radiation.[12] The first practical radio communications systems, developed by Guglielmo Marconi in 1894–1895, transmitted telegraph signals by radio waves,[4] so radio communication was first called "wireless telegraphy". Up until about 1910 the term "wireless telegraphy" also included a variety of other experimental systems for transmitting telegraph signals without wires, including electrostatic induction, electromagnetic induction and aquatic and earth conduction, so there was a need for a more precise term referring exclusively to electromagnetic radiation.[13][14]

The French physicist Édouard Branly, who in 1890 developed the radio wave detecting coherer, called it in French a radio-conducteur.[15][16] The radio- prefix was later used to form additional descriptive compound and hyphenated words, especially in Europe. For example, in early 1898 the British publication The Practical Engineer included a reference to "the radiotelegraph" and "radiotelegraphy".[15][17]

The use of "radio" as a standalone word dates back to at least December 30, 1904, when instructions issued by the British Post Office for transmitting telegrams specified that "The word 'Radio'... is sent in the Service Instructions".[15][18] This practice was universally adopted, and the word "radio" introduced internationally, by the 1906 Berlin Radiotelegraphic Convention, which included a Service Regulation specifying that "Radiotelegrams shall show in the preamble that the service is 'Radio'".[15]

The switch to "radio" in place of "wireless" took place slowly and unevenly in the English-speaking world. Lee de Forest helped popularize the new word in the United States—in early 1907, he founded the DeForest Radio Telephone Company, and his letter in the June 22, 1907 Electrical World about the need for legal restrictions warned that "Radio chaos will certainly be the result until such stringent regulation is enforced".[19] The United States Navy would also play a role. Although its translation of the 1906 Berlin Convention used the terms "wireless telegraph" and "wireless telegram", by 1912 it began to promote the use of "radio" instead. The term started to become preferred by the general public in the 1920s with the introduction of broadcasting.

History edit

See History of radio, Invention of radio, Timeline of radio, History of broadcasting

Electromagnetic waves were predicted by James Clerk Maxwell in his 1873 theory of electromagnetism, now called Maxwell's equations, who proposed that a coupled oscillating electric field and magnetic field could travel through space as a wave, and proposed that light consisted of electromagnetic waves of short wavelength. On November 11, 1886, German physicist Heinrich Hertz, attempting to confirm Maxwell's theory, first observed radio waves he generated using a primitive spark gap transmitter.[4] Experiments by Hertz and physicists Jagadish Chandra Bose, Oliver Lodge, Lord Rayleigh, and Augusto Righi, among others, showed that radio waves like light demonstrated reflection, refraction, diffraction, polarization, standing waves, and traveled at the same speed as light, confirming that both light and radio waves were electromagnetic waves, differing only in frequency.[20] In 1895, Guglielmo Marconi developed the first radio communication system, using a spark gap transmitter to send Morse code over long distances. By December 1901, he had transmitted across the Atlantic ocean.[4][5][6][7] Karl Ferdinand Braun invented the phased array antenna in 1905. Three antennas are arranged to transmit a directional signal.[21] Marconi and Braun shared the 1909 Nobel Prize in Physics "for their contributions to the development of wireless telegraphy".[22]

During radio's first two decades, called the radiotelegraphy era, the primitive damped wave radio transmitters could only transmit pulses of radio waves, not the continuous waves which were needed for audio modulation, so radio was used for person-to-person commercial, diplomatic and military text messaging. Starting around 1908 industrial countries built worldwide networks of powerful transoceanic spark transmitters to exchange telegram traffic between continents and communicate with their colonies and naval fleets. During World War 1 the development of continuous wave radio transmitters and rectifying electrolytic and crystal radio receiver detectors enabled amplitude modulation (AM) radiotelephony to be achieved by Reginald Fessenden and others, allowing sound (audio) to be transmitted. On November 2, 1920, the first commercial radio broadcast was transmitted by Westinghouse Electric and Manufacturing Company in Pittsburgh, under the call sign KDKA featuring live coverage of the Harding-Cox presidential election.[9]

Technology edit

Radio waves are radiated by electric charges undergoing acceleration.[23][24] They are generated artificially by time varying electric currents, consisting of electrons flowing back and forth in a metal conductor called an antenna.[25][26]

As they travel farther from the transmitting antenna, radio waves spread out so their signal strength (intensity in watts per square meter) decreases, so radio transmissions can only be received within a limited range of the transmitter, the distance depending on the transmitter power, the antenna radiation pattern, receiver sensitivity, noise level, and presence of obstructions between transmitter and receiver. An omnidirectional antenna transmits or receives radio waves in all directions, while a directional antenna or high-gain antenna transmits radio waves in a beam in a particular direction, or receives waves from only one direction.[27][28][29]

Radio waves travel at the speed of light in vacuum.[30][31]

The other types of electromagnetic waves besides radio waves, infrared, visible light, ultraviolet, X-rays and gamma rays, can also carry information and be used for communication. The wide use of radio waves for telecommunication is mainly due to their desirable propagation properties stemming from their large wavelength.[26]

Radio communication edit

 
Radio communication. Information such as sound is converted by a transducer such as a microphone to an electrical signal, which modulates a radio wave produced by the transmitter. A receiver intercepts the radio wave and extracts the information-bearing modulation signal, which is converted back to a human usable form with another transducer such as a loudspeaker.
 
Comparison of AM and FM modulated radio waves

In radio communication systems, information is carried across space using radio waves. At the sending end, the information to be sent is converted by some type of transducer to a time-varying electrical signal called the modulation signal.[26][32] The modulation signal may be an audio signal representing sound from a microphone, a video signal representing moving images from a video camera, or a digital signal consisting of a sequence of bits representing binary data from a computer. The modulation signal is applied to a radio transmitter. In the transmitter, an electronic oscillator generates an alternating current oscillating at a radio frequency, called the carrier wave because it serves to "carry" the information through the air. The information signal is used to modulate the carrier, varying some aspect of the carrier wave, impressing the information on the carrier. Different radio systems use different modulation methods:[33]

Many other types of modulation are also used. In some types, a carrier wave is not transmitted but just one or both modulation sidebands.[35]

The modulated carrier is amplified in the transmitter and applied to a transmitting antenna which radiates the energy as radio waves. The radio waves carry the information to the receiver location.[36] At the receiver, the radio wave induces a tiny oscillating voltage in the receiving antenna which is a weaker replica of the current in the transmitting antenna.[26][32] This voltage is applied to the radio receiver, which amplifies the weak radio signal so it is stronger, then demodulates it, extracting the original modulation signal from the modulated carrier wave. The modulation signal is converted by a transducer back to a human-usable form: an audio signal is converted to sound waves by a loudspeaker or earphones, a video signal is converted to images by a display, while a digital signal is applied to a computer or microprocessor, which interacts with human users.[33]

The radio waves from many transmitters pass through the air simultaneously without interfering with each other because each transmitter's radio waves oscillate at a different rate, in other words, each transmitter has a different frequency, measured in hertz (Hz), kilohertz (kHz), megahertz (MHz) or gigahertz (GHz). The receiving antenna typically picks up the radio signals of many transmitters. The receiver uses tuned circuits to select the radio signal desired out of all the signals picked up by the antenna and reject the others. A tuned circuit (also called resonant circuit or tank circuit) acts like a resonator, similar to a tuning fork.[32] It has a natural resonant frequency at which it oscillates. The resonant frequency of the receiver's tuned circuit is adjusted by the user to the frequency of the desired radio station; this is called "tuning". The oscillating radio signal from the desired station causes the tuned circuit to resonate, oscillate in sympathy, and it passes the signal on to the rest of the receiver. Radio signals at other frequencies are blocked by the tuned circuit and not passed on.[37]

Bandwidth edit

 
Frequency spectrum of a typical modulated AM or FM radio signal. It consists of a component C at the carrier wave frequency   with the information (modulation) contained in two narrow bands of frequencies called sidebands (SB) just above and below the carrier frequency.

A modulated radio wave, carrying an information signal, occupies a range of frequencies. The information (modulation) in a radio signal is usually concentrated in narrow frequency bands called sidebands (SB) just above and below the carrier frequency. The width in hertz of the frequency range that the radio signal occupies, the highest frequency minus the lowest frequency, is called its bandwidth (BW).[33][38] For any given signal-to-noise ratio, an amount of bandwidth can carry the same amount of information (data rate in bits per second) regardless of where in the radio frequency spectrum it is located, so bandwidth is a measure of information-carrying capacity. The bandwidth required by a radio transmission depends on the data rate of the information (modulation signal) being sent, and the spectral efficiency of the modulation method used; how much data it can transmit in each kilohertz of bandwidth. Different types of information signals carried by radio have different data rates. For example, a television (video) signal has a greater data rate than an audio signal.[33][39]

The radio spectrum, the total range of radio frequencies that can be used for communication in a given area, is a limited resource.[38][3] Each radio transmission occupies a portion of the total bandwidth available. Radio bandwidth is regarded as an economic good which has a monetary cost and is in increasing demand. In some parts of the radio spectrum, the right to use a frequency band or even a single radio channel is bought and sold for millions of dollars. So there is an incentive to employ technology to minimize the bandwidth used by radio services.[39]

A slow transition from analog to digital radio transmission technologies began in the late 1990s.[40][41] Part of the reason for this is that digital modulation can often transmit more information (a greater data rate) in a given bandwidth than analog modulation, by using data compression algorithms, which reduce redundancy in the data to be sent, and more efficient modulation. Other reasons for the transition is that digital modulation has greater noise immunity than analog, digital signal processing chips have more power and flexibility than analog circuits, and a wide variety of types of information can be transmitted using the same digital modulation.[33]

Because it is a fixed resource which is in demand by an increasing number of users, the radio spectrum has become increasingly congested in recent decades, and the need to use it more effectively is driving many additional radio innovations such as trunked radio systems, spread spectrum (ultra-wideband) transmission, frequency reuse, dynamic spectrum management, frequency pooling, and cognitive radio.[39]

ITU frequency bands edit

The ITU arbitrarily divides the radio spectrum into 12 bands, each beginning at a wavelength which is a power of ten (10n) metres, with corresponding frequency of 3 times a power of ten, and each covering a decade of frequency or wavelength.[3][42] Each of these bands has a traditional name:[43]

Band name Abbreviation Frequency Wavelength
Extremely
low frequency
ELF 3–30 Hz 100,000–
10,000 km
Super
low frequency
SLF 30–300 Hz 10,000 –
1,000 km
Ultra
low frequency
ULF 300–
3,000 Hz
1,000–
100 km
Very
low frequency
VLF 3–30 kHz 100–10 km
Low
frequency
LF 30–300 kHz 10–1 km
Medium
frequency
MF 300–
3,000 kHz
1,000–
100 m
Band name Abbreviation Frequency Wavelength
High
frequency
HF 3–30 MHz 100–10 m
Very
high frequency
VHF 30–300 MHz 10–1 m
Ultra
high frequency
UHF 300–
3,000 MHz
100–10 cm
Super
high frequency
SHF 3–30 GHz 10–1 cm
Extremely
high frequency
EHF 30–300 GHz 10–1 mm
Tremendously
high frequency
THF 300–3,000 GHz
(0.3–3.0 THz)
1.0–0.1 mm

It can be seen that the bandwidth, the range of frequencies, contained in each band is not equal but increases exponentially as the frequency increases; each band contains ten times the bandwidth of the preceding band.[44]

The term "tremendously low frequency" (TLF) has been used for wavelengths from 1–3 Hz (300,000–100,000 km),[45] but the term has not been defined by the ITU.[43]

Regulation edit

The airwaves are a resource shared by many users. Two radio transmitters in the same area that attempt to transmit on the same frequency will interfere with each other, causing garbled reception, so neither transmission may be received clearly.[38] Interference with radio transmissions can not only have a large economic cost, but it can also be life-threatening (for example, in the case of interference with emergency communications or air traffic control).[46][47]

To prevent interference between different users, the emission of radio waves is strictly regulated by national laws, coordinated by an international body, the International Telecommunication Union (ITU), which allocates bands in the radio spectrum for different uses.[38][3] Radio transmitters must be licensed by governments, under a variety of license classes depending on use, and are restricted to certain frequencies and power levels. In some classes, such as radio and television broadcasting stations, the transmitter is given a unique identifier consisting of a string of letters and numbers called a call sign, which must be used in all transmissions.[48] In order to adjust, maintain, or internally repair radiotelephone transmitters, individuals must hold a government license, such as the general radiotelephone operator license in the US, obtained by taking a test demonstrating adequate technical and legal knowledge of safe radio operation.[49]

Exceptions to the above rules allow the unlicensed operation by the public of low power short-range transmitters in consumer products such as cell phones, cordless phones, wireless devices, walkie-talkies, citizens band radios, wireless microphones, garage door openers, and baby monitors. In the US, these fall under Part 15 of the Federal Communications Commission (FCC) regulations. Many of these devices use the ISM bands, a series of frequency bands throughout the radio spectrum reserved for unlicensed use. Although they can be operated without a license, like all radio equipment these devices generally must be type-approved before the sale.[50]

Applications edit

Below are some of the most important uses of radio, organized by function.

Broadcasting edit

Broadcasting is the one-way transmission of information from a transmitter to receivers belonging to a public audience.[51] Since the radio waves become weaker with distance, a broadcasting station can only be received within a limited distance of its transmitter.[52] Systems that broadcast from satellites can generally be received over an entire country or continent. Older terrestrial radio and television are paid for by commercial advertising or governments. In subscription systems like satellite television and satellite radio the customer pays a monthly fee. In these systems, the radio signal is encrypted and can only be decrypted by the receiver, which is controlled by the company and can be deactivated if the customer does not pay.[53]

Broadcasting uses several parts of the radio spectrum, depending on the type of signals transmitted and the desired target audience. Longwave and medium wave signals can give reliable coverage of areas several hundred kilometers across, but have a more limited information-carrying capacity and so work best with audio signals (speech and music), and the sound quality can be degraded by radio noise from natural and artificial sources. The shortwave bands have a greater potential range but are more subject to interference by distant stations and varying atmospheric conditions that affect reception.[54][55]

In the very high frequency band, greater than 30 megahertz, the Earth's atmosphere has less of an effect on the range of signals, and line-of-sight propagation becomes the principal mode. These higher frequencies permit the great bandwidth required for television broadcasting. Since natural and artificial noise sources are less present at these frequencies, high-quality audio transmission is possible, using frequency modulation.[56][57]

Audio: Radio broadcasting edit

Radio broadcasting means transmission of audio (sound) to radio receivers belonging to a public audience. Analog audio is the earliest form of radio broadcast. AM broadcasting began around 1920. FM broadcasting was introduced in the late 1930s with improved fidelity. A broadcast radio receiver is called a radio. Most radios can receive both AM and FM.[58]

 
1100 W AM broadcasting transmitter
 
Mast radiator antenna of AM radio station
 
Panasonic AM radio from 1964
  • AM (amplitude modulation) – in AM, the amplitude (strength) of the radio carrier wave is varied by the audio signal. AM broadcasting, the oldest broadcasting technology, is allowed in the AM broadcast bands, between 148–283 kHz in the low frequency (LF) band for longwave broadcasts and between 526–1706 kHz in the medium frequency (MF) band for medium-wave broadcasts.[59] Because waves in these bands travel as ground waves following the terrain, AM radio stations can be received beyond the horizon at hundreds of miles distance, but AM has lower fidelity than FM. Radiated power (ERP) of AM stations in the US is usually limited to a maximum of 10 kW, although a few (clear-channel stations) are allowed to transmit at 50 kW. AM stations broadcast in monaural audio; AM stereo broadcast standards exist in most countries, but the radio industry has failed to upgrade to them, due to lack of demand.[60]
  • Shortwave broadcasting – AM broadcasting is also allowed in the shortwave bands by legacy radio stations. Since radio waves in these bands can travel intercontinental distances by reflecting off the ionosphere using skywave or "skip" propagation, shortwave is used by international stations, broadcasting to other countries.[60][61]
 
FM broadcast transmitter of radio station KWNR, Las Vegas, with a power of 35 kW on 95.5 MHz
 
FM broadcasting antenna
 
AM/FM boombox radio with FM whip antenna
  • FM (frequency modulation) – in FM the frequency of the radio carrier signal is varied slightly by the audio signal. FM broadcasting is permitted in the FM broadcast bands between about 65 and 108 MHz in the very high frequency (VHF) range. Radio waves in this band travel by line-of-sight so FM reception is limited by the visual horizon to about 30–40 mi (48–64 km), and can be blocked by hills. However it is less susceptible to interference from radio noise (RFI, sferics, static), and has higher fidelity, better frequency response, and less audio distortion than AM. In the US, radiated power (ERP) of FM stations varies from 6–100 kW.[62]
  • Digital radio involves a variety of standards and technologies for broadcasting digital radio signals over the air. Some systems, such as HD Radio and DRM, operate in the same wavebands as analog broadcasts, either as a replacement for analog stations or as a complementary service. Others, such as DAB/DAB+ and ISDB_Tsb, operate in wavebands traditionally used for television or satellite services.[63]
 
"Roberts" radio for DAB
  • Digital Audio Broadcasting (DAB) debuted in some countries in 1998. It transmits audio as a digital signal rather than an analog signal as AM and FM do.[64] DAB has the potential to provide higher quality sound than FM (although many stations do not choose to transmit at such high quality), has greater immunity to radio noise and interference, makes better use of scarce radio spectrum bandwidth and provides advanced user features such as electronic program guides. Its disadvantage is that it is incompatible with previous radios so that a new DAB receiver must be purchased.[65] Several nations have set dates to switch off analog FM networks in favor of DAB / DAB+, notably Norway in 2017[66] and Switzerland in 2024.[67]
A single DAB station transmits a 1,500 kHz bandwidth signal that carries from 9–12 channels of digital audio modulated by OFDM from which the listener can choose. Broadcasters can transmit a channel at a range of different bit rates, so different channels can have different audio quality. In different countries DAB stations broadcast in either Band III (174–240 MHz) or L band (1.452–1.492 GHz) in the UHF range, so like FM reception is limited by the visual horizon to about 40 miles (64 km).[68][65]
  • Digital Radio Mondiale (DRM) is a competing digital terrestrial radio standard developed mainly by broadcasters as a higher spectral efficiency replacement for legacy AM and FM broadcasting. Mondiale means "worldwide" in French and Italian; DRM was developed in 2001, and is currently supported by 23 countries, and adopted by some European and Eastern broadcasters beginning in 2003. The DRM30 mode uses the commercial broadcast bands below 30 MHz, and is intended as a replacement for standard AM broadcast on the longwave, mediumwave, and shortwave bands. The DRM+ mode uses VHF frequencies centered around the FM broadcast band, and is intended as a replacement for FM broadcasting. It is incompatible with existing radio receivers, so it requires listeners to purchase a new DRM receiver. The modulation used is a form of OFDM called COFDM in which, up to 4 carriers are transmitted on a channel formerly occupied by a single AM or FM signal, modulated by quadrature amplitude modulation (QAM).[73][61]
The DRM system is designed to be as compatible as possible with existing AM and FM radio transmitters, so that much of the equipment in existing radio stations can continue in use, augmented with DRM modulation equipment.[73][61]
 
Volkswagen's RNS-510 receiver supports Sirius Satellite Radio.

Video: Television broadcasting edit

Television broadcasting is the transmission of moving images by radio, which consist of sequences of still images, which are displayed on a screen on a television receiver (a "television" or TV) along with a synchronized audio (sound) channel. Television (video) signals occupy a wider bandwidth than broadcast radio (audio) signals. Analog television, the original television technology, required 6 MHz, so the television frequency bands are divided into 6 MHz channels, now called "RF channels".[76]

The current television standard, introduced beginning in 2006, is a digital format called high-definition television (HDTV), which transmits pictures at higher resolution, typically 1080 pixels high by 1920 pixels wide, at a rate of 25 or 30 frames per second. Digital television (DTV) transmission systems, which replaced older analog television in a transition beginning in 2006, use image compression and high-efficiency digital modulation such as OFDM and 8VSB to transmit HDTV video within a smaller bandwidth than the old analog channels, saving scarce radio spectrum space. Therefore, each of the 6 MHz analog RF channels now carries up to 7 DTV channels – these are called "virtual channels". Digital television receivers have different behavior in the presence of poor reception or noise than analog television, called the "digital cliff" effect. Unlike analog television, in which increasingly poor reception causes the picture quality to gradually degrade, in digital television picture quality is not affected by poor reception until, at a certain point, the receiver stops working and the screen goes black.[77][78]

 
Television studio control room, Celebro Studios, London
 
A television broadcasting antenna
 
A modern flatscreen television receiver
  • Terrestrial television, over-the-air (OTA) television, or broadcast television – the oldest television technology, is the transmission of television signals from land-based television stations to television receivers (called televisions or TVs) in viewer's homes. Terrestrial television broadcasting uses the bands 41 – 88 MHz (VHF low band or Band I, carrying RF channels 1–6), 174 – 240 MHz, (VHF high band or Band III; carrying RF channels 7–13), and 470 – 614 MHz (UHF Band IV and Band V; carrying RF channels 14 and up).[79] The exact frequency boundaries vary in different countries.[80] Propagation is by line-of-sight, so reception is limited by the visual horizon.[81] In the US, the effective radiated power (ERP) of television transmitters is regulated according to height above average terrain.[82] Viewers closer to the television transmitter can use a simple "rabbit ears" dipole antenna on top of the TV, but viewers in fringe reception areas typically require an outdoor antenna mounted on the roof to get adequate reception.[81]
 
 
(left) DISH Network's Super Dish 121 mounted on a rooftop. (right) A residential tower block with TV satellite dishes used by various users

Time edit

Government standard frequency and time signal services operate time radio stations which continuously broadcast extremely accurate time signals produced by atomic clocks, as a reference to synchronize other clocks.[85] Examples are BPC, DCF77, JJY, MSF, RTZ, TDF, WWV, and YVTO.[86] One use is in radio clocks and watches, which include an automated receiver that periodically (usually weekly) receives and decodes the time signal and resets the watch's internal quartz clock to the correct time, thus allowing a small watch or desk clock to have the same accuracy as an atomic clock. Government time stations are declining in number because GPS satellites and the Internet Network Time Protocol (NTP) provide equally accurate time standards.[87]

Two-way voice communication edit

 
 
(left) Cellphones, typical for Japan of the early 21st century. (right) Cellular phone tower shared by antennas belonging to 3 different networks.

A two-way radio is an audio transceiver, a receiver and transmitter in the same device, used for bidirectional person-to-person voice communication with other users with similar radios. An older term for this mode of communication is radiotelephony. The radio link may be half-duplex, as in a walkie-talkie, using a single radio channel in which only one radio can transmit at a time, so different users take turns talking, pressing a "push to talk" button on their radio which switches off the receiver and switches on the transmitter. Or the radio link may be full duplex, a bidirectional link using two radio channels so both people can talk at the same time, as in a cell phone.[88]

  • Cell phone – a portable wireless telephone that is connected to the telephone network by radio signals exchanged with a local antenna at a cellular base station (cell tower).[89] The service area covered by the provider is divided into small geographical areas called "cells", each served by a separate base station antenna and multichannel transceiver. All the cell phones in a cell communicate with this antenna on separate frequency channels, assigned from a common pool of frequencies. The purpose of cellular organization is to conserve radio bandwidth by frequency reuse. Low power transmitters are used so the radio waves used in a cell do not travel far beyond the cell, allowing the same frequencies to be reused in geographically separated cells. When a user carrying a cellphone crosses from one cell to another, his phone is automatically "handed off" seamlessly to the new antenna and assigned new frequencies. Cellphones have a highly automated full duplex digital transceiver using OFDM modulation using two digital radio channels, each carrying one direction of the bidirectional conversation, as well as a control channel that handles dialing calls and "handing off" the phone to another cell tower. Older 2G, 3G, and 4G networks use frequencies in the UHF and low microwave range, between 700 MHz and 3 GHz. The cell phone transmitter adjusts its power output to use the minimum power necessary to communicate with the cell tower; 0.6 W when near the tower, up to 3 W when farther away. Cell tower channel transmitter power is 50 W. Current generation phones, called smartphones, have many functions besides making telephone calls, and therefore have several other radio transmitters and receivers that connect them with other networks: usually a Wi-Fi modem, a Bluetooth modem, and a GPS receiver.[90][91][92]
 
 
(left) 5G millimeter wave antenna, Germany (right) Polish 5G smartphones
  • 5G cellular network – next-generation cellular networks which began deployment in 2019. Their major advantage is much higher data rates than previous cellular networks, up to 10 Gbps; 100 times faster than the previous cellular technology, 4G LTE. The higher data rates are achieved partly by using higher frequency radio waves, in the higher microwave band 3–6 GHz, and millimeter wave band, around 28 and 39 GHz. Since these frequencies have a shorter range than previous cellphone bands, the cells will be smaller than the cells in previous cellular networks which could be many miles across. Millimeter-wave cells will only be a few blocks long, and instead of a cell base station and antenna tower, they will have many small antennas attached to utility poles and buildings.[93][94]
 
Satellite phones, showing the large antennas needed to communicate with the satellite
  • Satellite phone (satphone) – a portable wireless telephone similar to a cell phone, connected to the telephone network through a radio link to an orbiting communications satellite instead of through cell towers. They are more expensive than cell phones; but their advantage is that, unlike a cell phone which is limited to areas covered by cell towers, satphones can be used over most or all of the geographical area of the Earth. In order for the phone to communicate with a satellite using a small omnidirectional antenna, first-generation systems use satellites in low Earth orbit, about 400–700 miles (640–1,100 km) above the surface. With an orbital period of about 100 minutes, a satellite can only be in view of a phone for about 4 – 15 minutes, so the call is "handed off" to another satellite when one passes beyond the local horizon. Therefore, large numbers of satellites, about 40 to 70, are required to ensure that at least one satellite is in view continuously from each point on Earth. Other satphone systems use satellites in geostationary orbit in which only a few satellites are needed, but these cannot be used at high latitudes because of terrestrial interference.[95][96]
  • Cordless phone – a landline telephone in which the handset is portable and communicates with the rest of the phone by a short-range full duplex radio link, instead of being attached by a cord. Both the handset and the base station have low-power radio transceivers that handle the short-range bidirectional radio link.[97] As of 2022, cordless phones in most nations use the DECT transmission standard.[98]
 
Motorola SCR-536 from WW2, the first walkie-talkie
  • Land mobile radio system – short-range mobile or portable half-duplex radio transceivers operating in the VHF or UHF band that can be used without a license. They are often installed in vehicles, with the mobile units communicating with a dispatcher at a fixed base station. Special systems with reserved frequencies are used by first responder services; police, fire, ambulance, and emergency services, and other government services. Other systems are made for use by commercial firms such as taxi and delivery services. VHF systems use channels in the range 30–50 MHz and 150–172 MHz. UHF systems use the 450–470 MHz band and in some areas the 470–512 MHz range. In general, VHF systems have a longer range than UHF but require longer antennas. AM or FM modulation is mainly used, but digital systems such as DMR are being introduced. The radiated power is typically limited to 4 watts.[89] These systems have a fairly limited range, usually 3 to 20 miles (4.8 to 32 km) depending on terrain. Repeaters installed on tall buildings, hills, or mountain peaks are often used to increase the range when it is desired to cover a larger area than line-of-sight. Examples of land mobile systems are CB, FRS, GMRS, and MURS. Modern digital systems, called trunked radio systems, have a digital channel management system using a control channel that automatically assigns frequency channels to user groups.[99]
 
Firefighter using modern walkie-talkie
    • Walkie-talkie – a battery-powered portable handheld half-duplex two-way radio, used in land mobile radio systems.[100]
  • Airband – Half-duplex radio system used by aircraft pilots to talk to other aircraft and ground-based air traffic controllers. This vital system is the main communication channel for air traffic control. For most communication in overland flights in air corridors a VHF-AM system using channels between 108 and 137 MHz in the VHF band is used. This system has a typical transmission range of 200 miles (320 km) for aircraft flying at cruising altitude.[101][102] For flights in more remote areas, such as transoceanic airline flights, aircraft use the HF band or channels on the Inmarsat or Iridium satphone satellites.[103] Military aircraft also use a dedicated UHF-AM band from 225.0 to 399.95 MHz.[104]
 
VHF marine radio on a ship
  • Marine radio – medium-range transceivers on ships, used for ship-to-ship, ship-to-air, and ship-to-shore communication with harbormasters They use FM channels between 156 and 174 MHz in the VHF band with up to 25 watts power, giving them a range of about 60 miles (97 km). Some channels are half-duplex and some are full-duplex, to be compatible with the telephone network, to allow users to make telephone calls through a marine operator.[105]
  • Amateur radio – long-range half-duplex two-way radio used by hobbyists for non-commercial purposes: recreational radio contacts with other amateurs, volunteer emergency communication during disasters, contests, and experimentation. Radio amateurs must hold an amateur radio license and are given a unique callsign that must be used as an identifier in transmissions. Amateur radio is restricted to small frequency bands, the amateur radio bands, spaced throughout the radio spectrum starting at 136 kHz. Within these bands, amateurs are allowed the freedom to transmit on any frequency using a wide variety of voice modulation methods, along with other forms of communication, such as slow-scan television (SSTV), and radioteletype (RTTY). Additionally, amateurs are among the only radio operators still using Morse code radiotelegraphy.[106]

One-way voice communication edit

One way, unidirectional radio transmission is called simplex.

  • Baby monitor – a crib-side appliance for parents of infants that transmits the baby's sounds to a receiver carried by the parent, so they can monitor the baby while they are in other parts of the house.[107] The wavebands used vary by region, but analog baby monitors generally transmit with low power in the 16, 9.3–49.9 or 900 MHz wavebands, and digital systems in the 2.4 GHz waveband.[108] Many baby monitors have duplex channels so the parent can talk to the baby, and cameras to show video of the baby.[109]
  • Wireless microphone – a battery-powered microphone with a short-range transmitter that is handheld or worn on a person's body which transmits its sound by radio to a nearby receiver unit connected to a sound system. Wireless microphones are used by public speakers, performers, and television personalities so they can move freely without trailing a microphone cord. Traditionally, analog models transmit in FM on unused portions of the television broadcast frequencies in the VHF and UHF bands. Some models transmit on two frequency channels for diversity reception to prevent nulls from interrupting transmission as the performer moves around.[110] Some models use digital modulation to prevent unauthorized reception by scanner radio receivers; these operate in the 900 MHz, 2.4 GHz or 6 GHz ISM bands.[111] European standards also support wireless multichannel audio systems (WMAS) that can better support the use of large numbers of wireless microphones at a single event or venue. As of 2021, U.S. regulators were considering adopting rules for WMAS.[112]

Data communication edit

  • Wireless networking – automated radio links which transmit digital data between computers and other wireless devices using radio waves, linking the devices together transparently in a computer network. Computer networks can transmit any form of data: in addition to email and web pages, they also carry phone calls (VoIP), audio, and video content (called streaming media). Security is more of an issue for wireless networks than for wired networks since anyone nearby with a wireless modem can access the signal and attempt to log in. The radio signals of wireless networks are encrypted using WPA.[113]
 
A laptop (with Wi-Fi module) and a typical home wireless router (on the right) connecting it to the Internet. The laptop shows its own photo
 
Neighborhood wireless WAN router on telephone pole
    • Wireless WAN (wireless wide area network, WWAN) – a variety of technologies that provide wireless internet access over a wider area than Wi-Fi networks do – from an office building to a campus to a neighborhood, or to an entire city. The most common technologies used are: cellular modems, that exchange computer data by radio with cell towers; satellite internet access; and lower frequencies in the UHF band, which have a longer range than Wi-Fi frequencies. Since WWAN networks are much more expensive and complicated to administer than Wi-Fi networks, their use so far has generally been limited to private networks operated by large corporations.[114]
    • Bluetooth – a very short-range wireless interface on a portable wireless device used as a substitute for a wire or cable connection, mainly to exchange files between portable devices and connect cellphones and music players with wireless headphones. In the most widely used mode, transmission power is limited to 1 milliwatt, giving it a very short range of up to 10 m (30 feet). The system uses frequency-hopping spread spectrum transmission, in which successive data packets are transmitted in a pseudorandom order on one of 79 1 MHz Bluetooth channels between 2.4 and 2.83 GHz in the ISM band. This allows Bluetooth networks to operate in the presence of noise, other wireless devices and other Bluetooth networks using the same frequencies, since the chance of another device attempting to transmit on the same frequency at the same time as the Bluetooth modem is low. In the case of such a "collision", the Bluetooth modem just retransmits the data packet on another frequency.[115]
    • Packet radio – a long-distance peer-to-peer wireless ad-hoc network in which data packets are exchanged between computer-controlled radio modems (transmitter/receivers) called nodes, which may be separated by miles, and maybe mobile. Each node only communicates with neighboring nodes, so packets of data are passed from node to node until they reach their destination using the X.25 network protocol. Packet radio systems are used to a limited degree by commercial telecommunications companies and by the amateur radio community.[116]
  • Text messaging (texting) – this is a service on cell phones, allowing a user to type a short alphanumeric message and send it to another phone number, and the text is displayed on the recipient's phone screen. It is based on the Short Message Service (SMS) which transmits using spare bandwidth on the control radio channel used by cell phones to handle background functions like dialing and cell handoffs. Due to technical limitations of the channel, text messages are limited to 160 alphanumeric characters.[117]
 
Parabolic antennas of microwave relay links on tower in Australia
  • Microwave relay – a long-distance high bandwidth point-to-point digital data transmission link consisting of a microwave transmitter connected to a dish antenna that transmits a beam of microwaves to another dish antenna and receiver. Since the antennas must be in line-of-sight, distances are limited by the visual horizon to 30–40 miles (48–64 km). Microwave links are used for private business data, wide area computer networks (WANs), and by telephone companies to transmit long-distance phone calls and television signals between cities.[118][119]
  • Telemetry – automated one-way (simplex) transmission of measurements and operation data from a remote process or device to a receiver for monitoring. Telemetry is used for in-flight monitoring of missiles, drones, satellites, and weather balloon radiosondes, sending scientific data back to Earth from interplanetary spacecraft, communicating with electronic biomedical sensors implanted in the human body, and well logging. Multiple channels of data are often transmitted using frequency-division multiplexing or time-division multiplexing.[120] Telemetry is starting to be used in consumer applications such as:
    • Automated meter readingelectric power meters, water meters, and gas meters that, when triggered by an interrogation signal, transmit their readings by radio to a utility reader vehicle at the curb, to eliminate the need for an employee to go on the customer's property to manually read the meter.[121]
    • Electronic toll collection – on toll roads, an alternative to manual collection of tolls at a toll booth, in which a transponder in a vehicle, when triggered by a roadside transmitter, transmits a signal to a roadside receiver to register the vehicle's use of the road, enabling the owner to be billed for the toll.[122]
 
RFID tag from a DVD
  • Radio Frequency Identification (RFID) – identification tags containing a tiny radio transponder (receiver and transmitter) which are attached to merchandise. When it receives an interrogation pulse of radio waves from a nearby reader unit, the tag transmits back an ID number, which can be used to inventory goods. Passive tags, the most common type, have a chip powered by the radio energy received from the reader, rectified by a diode, and can be as small as a grain of rice. They are incorporated in products, clothes, railroad cars, library books, airline baggage tags and are implanted under the skin in pets and livestock (microchip implant) and even people. Privacy concerns have been addressed with tags that use encrypted signals and authenticate the reader before responding. Passive tags use 125–134 kHz, 13, 900 MHz and 2.4 and 5 GHz ISM bands and have a short range. Active tags, powered by a battery, are larger but can transmit a stronger signal, giving them a range of hundreds of meters.[123]
  • Submarine communication – When submerged, submarines are cut off from all ordinary radio communication with their military command authorities by the conductive seawater. However radio waves of low enough frequencies, in the VLF (30 to 3 kHz) and ELF (below 3 kHz) bands are able to penetrate seawater. Navies operate large shore transmitting stations with power output in the megawatt range to transmit encrypted messages to their submarines in the world's oceans. Due to the small bandwidth, these systems cannot transmit voice, only text messages at a slow data rate. The communication channel is one-way, since the long antennas needed to transmit VLF or ELF waves cannot fit on a submarine. VLF transmitters use miles long wire antennas like umbrella antennas. A few nations use ELF transmitters operating around 80 Hz, which can communicate with submarines at lower depths. These use even larger antennas called ground dipoles, consisting of two ground (Earth) connections 23–60 km (14–37 mi) apart, linked by overhead transmission lines to a power plant transmitter.[124][125]

Space communication edit

 
Satellite Communications Center Dubna in Russia[126]

This is radio communication between a spacecraft and an Earth-based ground station, or another spacecraft. Communication with spacecraft involves the longest transmission distances of any radio links, up to billions of kilometers for interplanetary spacecraft. In order to receive the weak signals from distant spacecraft, satellite ground stations use large parabolic "dish" antennas up to 25 metres (82 ft) in diameter and extremely sensitive receivers. High frequencies in the microwave band are used, since microwaves pass through the ionosphere without refraction, and at microwave frequencies the high-gain antennas needed to focus the radio energy into a narrow beam pointed at the receiver are small and take up a minimum of space in a satellite. Portions of the UHF, L, C, S, ku and ka band are allocated for space communication. A radio link that transmits data from the Earth's surface to a spacecraft is called an uplink, while a link that transmits data from the spacecraft to the ground is called a downlink.[127]

 
Communications satellite belonging to Azerbaijan
  • Communication satellite – an artificial satellite used as a telecommunications relay to transmit data between widely separated points on Earth. These are used because the microwaves used for telecommunications travel by line of sight and so cannot propagate around the curve of the Earth. As of 1 January 2021, there were 2,224 communications satellites in Earth orbit.[128] Most are in geostationary orbit 22,200 miles (35,700 km) above the equator, so that the satellite appears stationary at the same point in the sky, so the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track it. In a satellite ground station a microwave transmitter and large satellite dish antenna transmit a microwave uplink beam to the satellite. The uplink signal carries many channels of telecommunications traffic, such as long-distance telephone calls, television programs, and internet signals, using a technique called frequency-division multiplexing (FDM). On the satellite, a transponder receives the signal, translates it to a different downlink frequency to avoid interfering with the uplink signal, and retransmits it down to another ground station, which may be widely separated from the first. There the downlink signal is demodulated and the telecommunications traffic it carries is sent to its local destinations through landlines. Communication satellites typically have several dozen transponders on different frequencies, which are leased by different users.[129]
  • Direct broadcast satellite – a geostationary communication satellite that transmits retail programming directly to receivers in subscriber's homes and vehicles on Earth, in satellite radio and TV systems. It uses a higher transmitter power than other communication satellites, to allow the signal to be received by consumers with a small unobtrusive antenna. For example, satellite television uses downlink frequencies from 12.2 to 12.7 GHz in the ku band transmitted at 100 to 250 watts, which can be received by relatively small 43–80 cm (17–31 in) satellite dishes mounted on the outside of buildings.[130]

Radar edit

 
Military air traffic controller on US Navy aircraft carrier monitors aircraft on radar screen

Radar is a radiolocation method used to locate and track aircraft, spacecraft, missiles, ships, vehicles, and also to map weather patterns and terrain. A radar set consists of a transmitter and receiver.[131][132] The transmitter emits a narrow beam of radio waves which is swept around the surrounding space. When the beam strikes a target object, radio waves are reflected back to the receiver. The direction of the beam reveals the object's location. Since radio waves travel at a constant speed close to the speed of light, by measuring the brief time delay between the outgoing pulse and the received "echo", the range to the target can be calculated. The targets are often displayed graphically on a map display called a radar screen. Doppler radar can measure a moving object's velocity, by measuring the change in frequency of the return radio waves due to the Doppler effect.[133]

Radar sets mainly use high frequencies in the microwave bands, because these frequencies create strong reflections from objects the size of vehicles and can be focused into narrow beams with compact antennas.[132] Parabolic (dish) antennas are widely used. In most radars the transmitting antenna also serves as the receiving antenna; this is called a monostatic radar. A radar which uses separate transmitting and receiving antennas is called a bistatic radar.[134]

 
ASR-8 airport surveillance radar antenna. It rotates once every 4.8 seconds. The rectangular antenna on top is the secondary radar.
  • Airport surveillance radar – In aviation, radar is the main tool of air traffic control. A rotating dish antenna sweeps a vertical fan-shaped beam of microwaves around the airspace and the radar set shows the location of aircraft as "blips" of light on a display called a radar screen. Airport radar operates at 2.7 – 2.9 GHz in the microwave S band. In large airports the radar image is displayed on multiple screens in an operations room called the TRACON (Terminal Radar Approach Control), where air traffic controllers direct the aircraft by radio to maintain safe aircraft separation.[135]
    • Secondary surveillance radar – Aircraft carry radar transponders, transceivers which when triggered by the incoming radar signal transmit a return microwave signal. This causes the aircraft to show up more strongly on the radar screen. The radar which triggers the transponder and receives the return beam, usually mounted on top of the primary radar dish, is called the secondary surveillance radar. Since radar cannot measure an aircraft's altitude with any accuracy, the transponder also transmits back the aircraft's altitude measured by its altimeter, and an ID number identifying the aircraft, which is displayed on the radar screen.[136]
  • Electronic countermeasures (ECM) – Military defensive electronic systems designed to degrade enemy radar effectiveness, or deceive it with false information, to prevent enemies from locating local forces. It often consists of powerful microwave transmitters that can mimic enemy radar signals to create false target indications on the enemy radar screens.[137]
 
Rotating marine radar antenna on a ship
  • Marine radar – an S or X band radar on ships used to detect nearby ships and obstructions like bridges.[138] A rotating antenna sweeps a vertical fan-shaped beam of microwaves around the water surface surrounding the craft out to the horizon.
  • Weather radar – A Doppler radar which maps weather precipitation intensities and wind speeds with the echoes returned from raindrops and their radial velocity by their Doppler shift.[139]
  • Phased-array radar – a radar set that uses a phased array, a computer-controlled antenna that can steer the radar beam quickly to point in different directions without moving the antenna. Phased-array radars were developed by the military to track fast-moving missiles and aircraft. They are widely used in military equipment and are now spreading to civilian applications.[140]
  • Synthetic aperture radar (SAR) – a specialized airborne radar set that produces a high-resolution map of ground terrain. The radar is mounted on an aircraft or spacecraft and the radar antenna radiates a beam of radio waves sideways at right angles to the direction of motion, toward the ground. In processing the return radar signal, the motion of the vehicle is used to simulate a large antenna, giving the radar a higher resolution.[141]
  • Ground-penetrating radar – a specialized radar instrument that is rolled along the ground surface in a cart and transmits a beam of radio waves into the ground, producing an image of subsurface objects. Frequencies from 100 MHz to a few GHz are used. Since radio waves cannot penetrate very far into earth, the depth of GPR is limited to about 50 feet.[142]
  • Collision avoidance system – a short range radar or LIDAR system on an automobile or vehicle that detects if the vehicle is about to collide with an object and applies the brakes to prevent the collision.[143]
  • Radar fuze – a detonator for an aerial bomb which uses a radar altimeter to measure the height of the bomb above the ground as it falls and detonates it at a certain altitude.[144]

Radiolocation edit

Radiolocation is a generic term covering a variety of techniques that use radio waves to find the location of objects, or for navigation.[145]

 
An early iPhone with its GPS navigation app in use.
 
A personal navigation assistant by Garmin, which uses GPS to give driving directions to a destination.
    • Global Positioning System (GPS) – The most widely used satellite navigation system, maintained by the US Air Force, which uses a constellation of 31 satellites in low Earth orbit. The orbits of the satellites are distributed so at any time at least four satellites are above the horizon over each point on Earth. Each satellite has an onboard atomic clock and transmits a continuous radio signal containing a precise time signal as well as its current position. Two frequencies are used, 1.2276 and 1.57542 GHz. Since the velocity of radio waves is virtually constant, the delay of the radio signal from a satellite is proportional to the distance of the receiver from the satellite. By receiving the signals from at least four satellites a GPS receiver can calculate its position on Earth by comparing the arrival time of the radio signals. Since each satellite's position is known precisely at any given time, from the delay the position of the receiver can be calculated by a microprocessor in the receiver. The position can be displayed as latitude and longitude, or as a marker on an electronic map. GPS receivers are incorporated in almost all cellphones and in vehicles such as automobiles, aircraft, and ships, and are used to guide drones, missiles, cruise missiles, and even artillery shells to their target, and handheld GPS receivers are produced for hikers and the military.[147][148]
  • Radio beacon – a fixed location terrestrial radio transmitter which transmits a continuous radio signal used by aircraft and ships for navigation. The locations of beacons are plotted on navigational maps used by aircraft and ships.[149]
    • VHF omnidirectional range (VOR) – a worldwide aircraft radio navigation system consisting of fixed ground radio beacons transmitting between 108.00 and 117.95 MHz in the very high frequency (VHF) band. An automated navigational instrument on the aircraft displays a bearing to a nearby VOR transmitter. A VOR beacon transmits two signals simultaneously on different frequencies. A directional antenna transmits a beam of radio waves that rotates like a lighthouse at a fixed rate, 30 times per second. When the directional beam is facing north, an omnidirectional antenna transmits a pulse. By measuring the difference in phase of these two signals, an aircraft can determine its bearing (or "radial") from the station accurately. By taking a bearing on two VOR beacons an aircraft can determine its position (called a "fix") to an accuracy of about 90 metres (300 ft). Most VOR beacons also have a distance measuring capability, called distance measuring equipment (DME); these are called VOR/DME's. The aircraft transmits a radio signal to the VOR/DME beacon and a transponder transmits a return signal. From the propagation delay between the transmitted and received signal the aircraft can calculate its distance from the beacon. This allows an aircraft to determine its location "fix" from only one VOR beacon. Since line-of-sight VHF frequencies are used VOR beacons have a range of about 200 miles for aircraft at cruising altitude. TACAN is a similar military radio beacon system which transmits in 962–1213 MHz, and a combined VOR and TACAN beacon is called a VORTAC. The number of VOR beacons is declining as aviation switches to the RNAV system that relies on Global Positioning System satellite navigation.[150][151]
    • Non-directional beacon (NDB) – Legacy fixed radio beacons used before the VOR system that transmit a simple signal in all directions for aircraft or ships to use for radio direction finding. Aircraft use automatic direction finder (ADF) receivers which use a directional antenna to determine the bearing to the beacon. By taking bearings on two beacons they can determine their position. NDBs use frequencies between 190 and 1750 kHz in the LF and MF bands which propagate beyond the horizon as ground waves or skywaves much farther than VOR beacons. They transmit a callsign consisting of one to 3 Morse code letters as an identifier.[152]
 
EPIRB emergency locator beacon on a ship
  • Emergency locator beacon – a portable battery powered radio transmitter used in emergencies to locate airplanes, vessels, and persons in distress and in need of immediate rescue. Various types of emergency locator beacons are carried by aircraft, ships, vehicles, hikers and cross-country skiers. In the event of an emergency, such as the aircraft crashing, the ship sinking, or a hiker becoming lost, the transmitter is deployed and begins to transmit a continuous radio signal, which is used by search and rescue teams to quickly find the emergency and render aid. The latest generation Emergency Position Indicating Rescue Beacons (EPIRBs) contain a GPS receiver, and broadcast to rescue teams their exact location within 20 meters.[153]
    • Cospas-Sarsat – an international humanitarian consortium of governmental and private agencies which acts as a dispatcher for search and rescue operations. It operates a network of about 47 satellites carrying radio receivers, which detect distress signals from emergency locator beacons anywhere on Earth transmitting on the international Cospas distress frequency of 406 MHz. The satellites calculate the geographic location of the beacon within 2 km by measuring the Doppler frequency shift of the radio waves due to the relative motion of the transmitter and the satellite, and quickly transmit the information to the appropriate local first responder organizations, which perform the search and rescue.[154][155]
 
Wildlife officer tracking radio-tagged mountain lion
  • Radio direction finding (RDF) – this is a general technique, used since the early 1900s, of using specialized radio receivers with directional antennas (RDF receivers) to determine the exact bearing of a radio signal, to determine the location of the transmitter. The location of a terrestrial transmitter can be determined by simple triangulation from bearings taken by two RDF stations separated geographically, as the point where the two bearing lines cross, this is called a "fix". Military forces use RDF to locate enemy forces by their tactical radio transmissions, counterintelligence services use it to locate clandestine transmitters used by espionage agents, and governments use it to locate unlicensed transmitters or interference sources. Older RDF receivers used rotatable loop antennas, the antenna is rotated until the radio signal strength is weakest, indicating the transmitter is in one of the antenna's two nulls. The nulls are used since they are sharper than the antenna's lobes (maxima). More modern receivers use phased array antennas which have a much greater angular resolution.[156][157]
    • Animal migration tracking – a widely used technique in wildlife biology, conservation biology, and wildlife management in which small battery-powered radio transmitters are attached to wild animals so their movements can be tracked with a directional RDF receiver. Sometimes the transmitter is implanted in the animal. The VHF band is typically used since antennas in this band are fairly compact. The receiver has a directional antenna (typically a small Yagi) which is rotated until the received signal is strongest; at this point the antenna is pointing in the direction of the animal. Sophisticated systems used in recent years use satellites to track the animal, or geolocation tags with GPS receivers which record and transmit a log of the animal's location.[158]

Remote control edit

 
US Air Force MQ-1 Predator drone flown remotely by a pilot on the ground

Radio remote control is the use of electronic control signals sent by radio waves from a transmitter to control the actions of a device at a remote location. Remote control systems may also include telemetry channels in the other direction, used to transmit real-time information on the state of the device back to the control station. Uncrewed spacecraft are an example of remote-controlled machines, controlled by commands transmitted by satellite ground stations. Most handheld remote controls used to control consumer electronics products like televisions or DVD players actually operate by infrared light rather than radio waves, so are not examples of radio remote control. A security concern with remote control systems is spoofing, in which an unauthorized person transmits an imitation of the control signal to take control of the device.[159] Examples of radio remote control:

  • Unmanned aerial vehicle (UAV, drone) – A drone is an aircraft without an onboard pilot, flown by remote control by a pilot in another location, usually in a piloting station on the ground. They are used by the military for reconnaissance and ground attack, and more recently by the civilian world for news reporting and aerial photography. The pilot uses aircraft controls like a joystick or steering wheel, which create control signals which are transmitted to the drone by radio to control the flight surfaces and engine. A telemetry system transmits back a video image from a camera in the drone to allow the pilot to see where the aircraft is going, and data from a GPS receiver giving the real-time position of the aircraft. UAVs have sophisticated onboard automatic pilot systems that maintain stable flight and only require manual control to change directions.[160]
 
Remote keyless entry fob for a car
  • Keyless entry system – a short-range handheld battery powered key fob transmitter, included with most modern cars, which can lock and unlock the doors of a vehicle from outside, eliminating the need to use a key. When a button is pressed, the transmitter sends a coded radio signal to a receiver in the vehicle, operating the locks. The fob must be close to the vehicle, typically within 5 to 20 meters. North America and Japan use a frequency of 315 MHz, while Europe uses 433.92 and 868 MHz. Some models can also remotely start the engine, to warm up the car. A security concern with all keyless entry systems is a replay attack, in which a thief uses a special receiver ("code grabber") to record the radio signal during opening, which can later be replayed to open the door. To prevent this, keyless systems use a rolling code system in which a pseudorandom number generator in the remote control generates a different random key each time it is used. To prevent thieves from simulating the pseudorandom generator to calculate the next key, the radio signal is also encrypted.[161]
    • Garage door opener – a short-range handheld transmitter which can open or close a building's electrically operated garage door from outside, so the owner can open the door upon arrival, and close it after departure. When a button is pressed the control transmits a coded FSK radio signal to a receiver in the opener, raising or lowering the door. Modern openers use 310, 315 or 390 MHz. To prevent a thief using a replay attack, modern openers use a rolling code system.[162][163]
 
Quadcopter, a popular remote-controlled toy
  • Radio-controlled models – a popular hobby is playing with radio-controlled model boats, cars, airplanes, and helicopters (quadcopters) which are controlled by radio signals from a handheld console with a joystick. Most recent transmitters use the 2.4 GHz ISM band with multiple control channels modulated with PWM, PCM or FSK.[164]
  • Wireless doorbell – A residential doorbell that uses wireless technology to eliminate the need to run wires through the building walls. It consists of a doorbell button beside the door containing a small battery powered transmitter. When the doorbell is pressed it sends a signal to a receiver inside the house with a speaker that sounds chimes to indicate someone is at the door. They usually use the 2.4 GHz ISM band. The frequency channel used can usually be changed by the owner in case another nearby doorbell is using the same channel.[165][166]

Jamming edit

Radio jamming is the deliberate radiation of radio signals designed to interfere with the reception of other radio signals. Jamming devices are called "signal suppressors" or "interference generators" or just jammers.[167]

During wartime, militaries use jamming to interfere with enemies' tactical radio communication. Since radio waves can pass beyond national borders, some totalitarian countries which practice censorship use jamming to prevent their citizens from listening to broadcasts from radio stations in other countries. Jamming is usually accomplished by a powerful transmitter which generates noise on the same frequency as the target transmitter.[168][169]

US Federal law prohibits the nonmilitary operation or sale of any type of jamming devices, including ones that interfere with GPS, cellular, Wi-Fi and police radars.[170]

Scientific research edit

  • Radio astronomy is the scientific study of radio waves emitted by astronomical objects. Radio astronomers use radio telescopes, large radio antennas and receivers, to receive and study the radio waves from astronomical radio sources. Since astronomical radio sources are so far away, the radio waves from them are extremely weak, requiring extremely sensitive receivers, and radio telescopes are the most sensitive radio receivers in existence. They use large parabolic (dish) antennas up to 500 meters (2,000 ft) in diameter to collect enough radio wave energy to study. The RF front end electronics of the receiver is often cooled by liquid nitrogen to reduce thermal noise. Multiple antennas are often linked together in arrays which function as a single antenna, to increase collecting power. In Very Long Baseline Interferometry (VLBI) radio telescopes on different continents are linked, which can achieve the resolution of an antenna thousands of miles in diameter.[171][172]
  • Remote sensing – in radio, remote sensing is the reception of electromagnetic waves radiated by natural objects or the atmosphere for scientific research. All warm objects emit microwaves and the spectrum emitted can be used to determine temperature. Microwave radiometers are used in meteorology and earth sciences to determine temperature of the atmosphere and earth surface, as well as chemical reactions in the atmosphere.[173][174]

See also edit

References edit

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  2. ^ "Definition of radio". Encyclopedia. PCMagazine website, Ziff-Davis. 2018. Retrieved 26 February 2019.
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  5. ^ a b Bondyopadhyay, Prebir K. (1995) "Guglielmo Marconi – The father of long distance radio communication – An engineer's tribute", 25th European Microwave Conference: Volume 2, pp. 879–85
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  9. ^ a b "History of Commercial Radio". Federal Communications Commission. 23 October 2020. from the original on January 1, 2022. Retrieved July 14, 2022.
  10. ^ "radio (n.)". Online Etymology Dictionary. Retrieved July 13, 2022.
  11. ^ Bell, Alexander Graham (July 1881). "Production of Sound by Radiant Energy". Popular Science Monthly. pp. 329–330. [W]e have named the apparatus for the production and reproduction of sound in this way the "photophone", because an ordinary beam of light contains the rays which are operative. To avoid in future any misunderstandings upon this point, we have decided to adopt the term "radiophone", proposed by M. Mercadier, as a general term signifying the production of sound by any form of radiant energy...
  12. ^ Manning, Trevor (2009). Microwave Radio Transmission Design Guide. Artech House. p. 2.
  13. ^ Maver, William Jr. (1903). American Telegraphy and Encyclopedia of the Telegraph: Systems, Apparatus, Operation. New York: Maver Publishing Co. p. 333. wireless telegraphy.
  14. ^ Steuart, William Mott; et al. (1906). Special Reports: Telephones and Telegraphs 1902. Washington D.C.: U.S. Bureau of the Census. pp. 118–119.
  15. ^ a b c d https://earlyradiohistory.us/sec022.htm Thomas H. White, United States Early Radio History, Section 22
  16. ^ Collins, A. Frederick (May 10, 1902). "The Genesis of Wireless Telegraphy". Electrical World and Engineer. p. 811.
  17. ^ "Wireless Telegraphy". The Practical Engineer. February 25, 1898. p. 174. Dr. O. J. Lodge, who preceded Marconi in making experiments in what may be called "ray" telegraphy or radiotelegraphy by a year or two, has devised a new method of sending and receiving the messages. The reader will understand that in the radiotelegraph electric waves forming the signals of the message starting from the sending instrument and travel in all directions like rays of light from a lamp, only they are invisible.
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  19. ^ "Interference with Wireless Messages", Electrical World, June 22, 1907, page 1270.
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General references edit

  • Basic Radio Principles and Technology – Elsevier Science
  • The Electronics of Radio – Cambridge University Press
  • Radio Systems Engineering – Cambridge University Press
  • Radio-Electronic Transmission Fundamentals – SciTech Publishing
  • Analog Electronics, Analog Circuitry Explained – Elsevier Science

External links edit

radio, this, article, about, science, technology, audio, distribution, broadcasting, user, device, receiver, other, uses, disambiguation, technology, signaling, communicating, using, radio, waves, waves, electromagnetic, waves, frequency, between, hertz, gigah. This article is about science and technology For its use in audio distribution see Radio broadcasting For the user device see Radio receiver For other uses see Radio disambiguation Radio is the technology of signaling and communicating using radio waves 1 2 3 Radio waves are electromagnetic waves of frequency between 3 hertz Hz and 3 000 gigahertz GHz They are generated by an electronic device called a transmitter connected to an antenna which radiates the waves and received by another antenna connected to a radio receiver Radio is widely used in modern technology in radio communication radar radio navigation remote control remote sensing and other applications A variety of radio antennas on Sandia Peak near Albuquerque New Mexico USIn radio communication used in radio and television broadcasting cell phones two way radios wireless networking and satellite communication among numerous other uses radio waves are used to carry information across space from a transmitter to a receiver by modulating the radio signal impressing an information signal on the radio wave by varying some aspect of the wave in the transmitter In radar used to locate and track objects like aircraft ships spacecraft and missiles a beam of radio waves emitted by a radar transmitter reflects off the target object and the reflected waves reveal the object s location In radio navigation systems such as GPS and VOR a mobile navigation instrument receives radio signals from navigational radio beacons whose position is known and by precisely measuring the arrival time of the radio waves the receiver can calculate its position on Earth In wireless radio remote control devices like drones garage door openers and keyless entry systems radio signals transmitted from a controller device control the actions of a remote device The noun radio is also used to mean a broadcast radio receiver The existence of radio waves was first proven by German physicist Heinrich Hertz on November 11 1886 4 In the mid 1890s building on techniques physicists were using to study electromagnetic waves Guglielmo Marconi developed the first apparatus for long distance radio communication 5 sending a wireless Morse Code message to a recipient over a kilometer away in 1895 6 and the first transatlantic signal on December 12 1901 7 Karl Ferdinand Braun invented the phased array antenna in 1905 8 The first commercial radio broadcast was transmitted on November 2 1920 when the live returns of the Harding Cox presidential election were broadcast by Westinghouse Electric and Manufacturing Company in Pittsburgh under the call sign KDKA 9 The emission of radio waves is regulated by law coordinated by the International Telecommunication Union ITU which allocates frequency bands in the radio spectrum for various uses Contents 1 Etymology 2 History 3 Technology 4 Radio communication 4 1 Bandwidth 4 2 ITU frequency bands 5 Regulation 6 Applications 6 1 Broadcasting 6 1 1 Audio Radio broadcasting 6 1 2 Video Television broadcasting 6 1 3 Time 6 2 Two way voice communication 6 3 One way voice communication 6 4 Data communication 6 5 Space communication 6 6 Radar 6 7 Radiolocation 6 8 Remote control 6 9 Jamming 6 10 Scientific research 7 See also 8 References 9 General references 10 External linksEtymology editThe word radio is derived from the Latin word radius meaning spoke of a wheel beam of light ray It was first applied to communications in 1881 when at the suggestion of French scientist Ernest Mercadier fr Alexander Graham Bell adopted radiophone meaning radiated sound as an alternate name for his photophone optical transmission system 10 11 Following Heinrich Hertz s discovery of the existence of radio waves in 1886 the term Hertzian waves was initially used for this radiation 12 The first practical radio communications systems developed by Guglielmo Marconi in 1894 1895 transmitted telegraph signals by radio waves 4 so radio communication was first called wireless telegraphy Up until about 1910 the term wireless telegraphy also included a variety of other experimental systems for transmitting telegraph signals without wires including electrostatic induction electromagnetic induction and aquatic and earth conduction so there was a need for a more precise term referring exclusively to electromagnetic radiation 13 14 The French physicist Edouard Branly who in 1890 developed the radio wave detecting coherer called it in French a radio conducteur 15 16 The radio prefix was later used to form additional descriptive compound and hyphenated words especially in Europe For example in early 1898 the British publication The Practical Engineer included a reference to the radiotelegraph and radiotelegraphy 15 17 The use of radio as a standalone word dates back to at least December 30 1904 when instructions issued by the British Post Office for transmitting telegrams specified that The word Radio is sent in the Service Instructions 15 18 This practice was universally adopted and the word radio introduced internationally by the 1906 Berlin Radiotelegraphic Convention which included a Service Regulation specifying that Radiotelegrams shall show in the preamble that the service is Radio 15 The switch to radio in place of wireless took place slowly and unevenly in the English speaking world Lee de Forest helped popularize the new word in the United States in early 1907 he founded the DeForest Radio Telephone Company and his letter in the June 22 1907 Electrical World about the need for legal restrictions warned that Radio chaos will certainly be the result until such stringent regulation is enforced 19 The United States Navy would also play a role Although its translation of the 1906 Berlin Convention used the terms wireless telegraph and wireless telegram by 1912 it began to promote the use of radio instead The term started to become preferred by the general public in the 1920s with the introduction of broadcasting History editSee History of radio Invention of radio Timeline of radio History of broadcastingElectromagnetic waves were predicted by James Clerk Maxwell in his 1873 theory of electromagnetism now called Maxwell s equations who proposed that a coupled oscillating electric field and magnetic field could travel through space as a wave and proposed that light consisted of electromagnetic waves of short wavelength On November 11 1886 German physicist Heinrich Hertz attempting to confirm Maxwell s theory first observed radio waves he generated using a primitive spark gap transmitter 4 Experiments by Hertz and physicists Jagadish Chandra Bose Oliver Lodge Lord Rayleigh and Augusto Righi among others showed that radio waves like light demonstrated reflection refraction diffraction polarization standing waves and traveled at the same speed as light confirming that both light and radio waves were electromagnetic waves differing only in frequency 20 In 1895 Guglielmo Marconi developed the first radio communication system using a spark gap transmitter to send Morse code over long distances By December 1901 he had transmitted across the Atlantic ocean 4 5 6 7 Karl Ferdinand Braun invented the phased array antenna in 1905 Three antennas are arranged to transmit a directional signal 21 Marconi and Braun shared the 1909 Nobel Prize in Physics for their contributions to the development of wireless telegraphy 22 During radio s first two decades called the radiotelegraphy era the primitive damped wave radio transmitters could only transmit pulses of radio waves not the continuous waves which were needed for audio modulation so radio was used for person to person commercial diplomatic and military text messaging Starting around 1908 industrial countries built worldwide networks of powerful transoceanic spark transmitters to exchange telegram traffic between continents and communicate with their colonies and naval fleets During World War 1 the development of continuous wave radio transmitters and rectifying electrolytic and crystal radio receiver detectors enabled amplitude modulation AM radiotelephony to be achieved by Reginald Fessenden and others allowing sound audio to be transmitted On November 2 1920 the first commercial radio broadcast was transmitted by Westinghouse Electric and Manufacturing Company in Pittsburgh under the call sign KDKA featuring live coverage of the Harding Cox presidential election 9 Technology editRadio waves are radiated by electric charges undergoing acceleration 23 24 They are generated artificially by time varying electric currents consisting of electrons flowing back and forth in a metal conductor called an antenna 25 26 As they travel farther from the transmitting antenna radio waves spread out so their signal strength intensity in watts per square meter decreases so radio transmissions can only be received within a limited range of the transmitter the distance depending on the transmitter power the antenna radiation pattern receiver sensitivity noise level and presence of obstructions between transmitter and receiver An omnidirectional antenna transmits or receives radio waves in all directions while a directional antenna or high gain antenna transmits radio waves in a beam in a particular direction or receives waves from only one direction 27 28 29 Radio waves travel at the speed of light in vacuum 30 31 The other types of electromagnetic waves besides radio waves infrared visible light ultraviolet X rays and gamma rays can also carry information and be used for communication The wide use of radio waves for telecommunication is mainly due to their desirable propagation properties stemming from their large wavelength 26 Radio communication edit nbsp Radio communication Information such as sound is converted by a transducer such as a microphone to an electrical signal which modulates a radio wave produced by the transmitter A receiver intercepts the radio wave and extracts the information bearing modulation signal which is converted back to a human usable form with another transducer such as a loudspeaker nbsp Comparison of AM and FM modulated radio wavesIn radio communication systems information is carried across space using radio waves At the sending end the information to be sent is converted by some type of transducer to a time varying electrical signal called the modulation signal 26 32 The modulation signal may be an audio signal representing sound from a microphone a video signal representing moving images from a video camera or a digital signal consisting of a sequence of bits representing binary data from a computer The modulation signal is applied to a radio transmitter In the transmitter an electronic oscillator generates an alternating current oscillating at a radio frequency called the carrier wave because it serves to carry the information through the air The information signal is used to modulate the carrier varying some aspect of the carrier wave impressing the information on the carrier Different radio systems use different modulation methods 33 AM amplitude modulation in an AM transmitter the amplitude strength of the radio carrier wave is varied by the modulation signal 33 3 FM frequency modulation in an FM transmitter the frequency of the radio carrier wave is varied by the modulation signal 33 33 FSK frequency shift keying used in wireless digital devices to transmit digital signals the frequency of the carrier wave is shifted between frequencies 33 58 OFDM orthogonal frequency division multiplexing a family of digital modulation methods widely used in high bandwidth systems such as Wi Fi networks cellphones digital television broadcasting and digital audio broadcasting DAB to transmit digital data using a minimum of radio spectrum bandwidth It has higher spectral efficiency and more resistance to fading than AM or FM In OFDM multiple radio carrier waves closely spaced in frequency are transmitted within the radio channel with each carrier modulated with bits from the incoming bitstream so multiple bits are being sent simultaneously in parallel At the receiver the carriers are demodulated and the bits are combined in the proper order into one bitstream 34 Many other types of modulation are also used In some types a carrier wave is not transmitted but just one or both modulation sidebands 35 The modulated carrier is amplified in the transmitter and applied to a transmitting antenna which radiates the energy as radio waves The radio waves carry the information to the receiver location 36 At the receiver the radio wave induces a tiny oscillating voltage in the receiving antenna which is a weaker replica of the current in the transmitting antenna 26 32 This voltage is applied to the radio receiver which amplifies the weak radio signal so it is stronger then demodulates it extracting the original modulation signal from the modulated carrier wave The modulation signal is converted by a transducer back to a human usable form an audio signal is converted to sound waves by a loudspeaker or earphones a video signal is converted to images by a display while a digital signal is applied to a computer or microprocessor which interacts with human users 33 The radio waves from many transmitters pass through the air simultaneously without interfering with each other because each transmitter s radio waves oscillate at a different rate in other words each transmitter has a different frequency measured in hertz Hz kilohertz kHz megahertz MHz or gigahertz GHz The receiving antenna typically picks up the radio signals of many transmitters The receiver uses tuned circuits to select the radio signal desired out of all the signals picked up by the antenna and reject the others A tuned circuit also called resonant circuit or tank circuit acts like a resonator similar to a tuning fork 32 It has a natural resonant frequency at which it oscillates The resonant frequency of the receiver s tuned circuit is adjusted by the user to the frequency of the desired radio station this is called tuning The oscillating radio signal from the desired station causes the tuned circuit to resonate oscillate in sympathy and it passes the signal on to the rest of the receiver Radio signals at other frequencies are blocked by the tuned circuit and not passed on 37 Bandwidth edit nbsp Frequency spectrum of a typical modulated AM or FM radio signal It consists of a component C at the carrier wave frequency f c displaystyle f c nbsp with the information modulation contained in two narrow bands of frequencies called sidebands SB just above and below the carrier frequency A modulated radio wave carrying an information signal occupies a range of frequencies The information modulation in a radio signal is usually concentrated in narrow frequency bands called sidebands SB just above and below the carrier frequency The width in hertz of the frequency range that the radio signal occupies the highest frequency minus the lowest frequency is called its bandwidth BW 33 38 For any given signal to noise ratio an amount of bandwidth can carry the same amount of information data rate in bits per second regardless of where in the radio frequency spectrum it is located so bandwidth is a measure of information carrying capacity The bandwidth required by a radio transmission depends on the data rate of the information modulation signal being sent and the spectral efficiency of the modulation method used how much data it can transmit in each kilohertz of bandwidth Different types of information signals carried by radio have different data rates For example a television video signal has a greater data rate than an audio signal 33 39 The radio spectrum the total range of radio frequencies that can be used for communication in a given area is a limited resource 38 3 Each radio transmission occupies a portion of the total bandwidth available Radio bandwidth is regarded as an economic good which has a monetary cost and is in increasing demand In some parts of the radio spectrum the right to use a frequency band or even a single radio channel is bought and sold for millions of dollars So there is an incentive to employ technology to minimize the bandwidth used by radio services 39 A slow transition from analog to digital radio transmission technologies began in the late 1990s 40 41 Part of the reason for this is that digital modulation can often transmit more information a greater data rate in a given bandwidth than analog modulation by using data compression algorithms which reduce redundancy in the data to be sent and more efficient modulation Other reasons for the transition is that digital modulation has greater noise immunity than analog digital signal processing chips have more power and flexibility than analog circuits and a wide variety of types of information can be transmitted using the same digital modulation 33 Because it is a fixed resource which is in demand by an increasing number of users the radio spectrum has become increasingly congested in recent decades and the need to use it more effectively is driving many additional radio innovations such as trunked radio systems spread spectrum ultra wideband transmission frequency reuse dynamic spectrum management frequency pooling and cognitive radio 39 ITU frequency bands edit The ITU arbitrarily divides the radio spectrum into 12 bands each beginning at a wavelength which is a power of ten 10n metres with corresponding frequency of 3 times a power of ten and each covering a decade of frequency or wavelength 3 42 Each of these bands has a traditional name 43 Band name Abbreviation Frequency WavelengthExtremelylow frequency ELF 3 30 Hz 100 000 10 000 kmSuperlow frequency SLF 30 300 Hz 10 000 1 000 kmUltralow frequency ULF 300 3 000 Hz 1 000 100 kmVerylow frequency VLF 3 30 kHz 100 10 kmLowfrequency LF 30 300 kHz 10 1 kmMediumfrequency MF 300 3 000 kHz 1 000 100 m Band name Abbreviation Frequency WavelengthHighfrequency HF 3 30 MHz 100 10 mVeryhigh frequency VHF 30 300 MHz 10 1 mUltrahigh frequency UHF 300 3 000 MHz 100 10 cmSuperhigh frequency SHF 3 30 GHz 10 1 cmExtremelyhigh frequency EHF 30 300 GHz 10 1 mmTremendouslyhigh frequency THF 300 3 000 GHz 0 3 3 0 THz 1 0 0 1 mm It can be seen that the bandwidth the range of frequencies contained in each band is not equal but increases exponentially as the frequency increases each band contains ten times the bandwidth of the preceding band 44 The term tremendously low frequency TLF has been used for wavelengths from 1 3 Hz 300 000 100 000 km 45 but the term has not been defined by the ITU 43 Regulation editFurther information Radio regulation and Radio communication service The airwaves are a resource shared by many users Two radio transmitters in the same area that attempt to transmit on the same frequency will interfere with each other causing garbled reception so neither transmission may be received clearly 38 Interference with radio transmissions can not only have a large economic cost but it can also be life threatening for example in the case of interference with emergency communications or air traffic control 46 47 To prevent interference between different users the emission of radio waves is strictly regulated by national laws coordinated by an international body the International Telecommunication Union ITU which allocates bands in the radio spectrum for different uses 38 3 Radio transmitters must be licensed by governments under a variety of license classes depending on use and are restricted to certain frequencies and power levels In some classes such as radio and television broadcasting stations the transmitter is given a unique identifier consisting of a string of letters and numbers called a call sign which must be used in all transmissions 48 In order to adjust maintain or internally repair radiotelephone transmitters individuals must hold a government license such as the general radiotelephone operator license in the US obtained by taking a test demonstrating adequate technical and legal knowledge of safe radio operation 49 Exceptions to the above rules allow the unlicensed operation by the public of low power short range transmitters in consumer products such as cell phones cordless phones wireless devices walkie talkies citizens band radios wireless microphones garage door openers and baby monitors In the US these fall under Part 15 of the Federal Communications Commission FCC regulations Many of these devices use the ISM bands a series of frequency bands throughout the radio spectrum reserved for unlicensed use Although they can be operated without a license like all radio equipment these devices generally must be type approved before the sale 50 Applications editBelow are some of the most important uses of radio organized by function Broadcasting edit Further information Broadcasting Broadcasting is the one way transmission of information from a transmitter to receivers belonging to a public audience 51 Since the radio waves become weaker with distance a broadcasting station can only be received within a limited distance of its transmitter 52 Systems that broadcast from satellites can generally be received over an entire country or continent Older terrestrial radio and television are paid for by commercial advertising or governments In subscription systems like satellite television and satellite radio the customer pays a monthly fee In these systems the radio signal is encrypted and can only be decrypted by the receiver which is controlled by the company and can be deactivated if the customer does not pay 53 Broadcasting uses several parts of the radio spectrum depending on the type of signals transmitted and the desired target audience Longwave and medium wave signals can give reliable coverage of areas several hundred kilometers across but have a more limited information carrying capacity and so work best with audio signals speech and music and the sound quality can be degraded by radio noise from natural and artificial sources The shortwave bands have a greater potential range but are more subject to interference by distant stations and varying atmospheric conditions that affect reception 54 55 In the very high frequency band greater than 30 megahertz the Earth s atmosphere has less of an effect on the range of signals and line of sight propagation becomes the principal mode These higher frequencies permit the great bandwidth required for television broadcasting Since natural and artificial noise sources are less present at these frequencies high quality audio transmission is possible using frequency modulation 56 57 Audio Radio broadcasting edit Main article Radio broadcasting Radio broadcasting means transmission of audio sound to radio receivers belonging to a public audience Analog audio is the earliest form of radio broadcast AM broadcasting began around 1920 FM broadcasting was introduced in the late 1930s with improved fidelity A broadcast radio receiver is called a radio Most radios can receive both AM and FM 58 nbsp 1100 W AM broadcasting transmitter nbsp Mast radiator antenna of AM radio station nbsp Panasonic AM radio from 1964 AM amplitude modulation in AM the amplitude strength of the radio carrier wave is varied by the audio signal AM broadcasting the oldest broadcasting technology is allowed in the AM broadcast bands between 148 283 kHz in the low frequency LF band for longwave broadcasts and between 526 1706 kHz in the medium frequency MF band for medium wave broadcasts 59 Because waves in these bands travel as ground waves following the terrain AM radio stations can be received beyond the horizon at hundreds of miles distance but AM has lower fidelity than FM Radiated power ERP of AM stations in the US is usually limited to a maximum of 10 kW although a few clear channel stations are allowed to transmit at 50 kW AM stations broadcast in monaural audio AM stereo broadcast standards exist in most countries but the radio industry has failed to upgrade to them due to lack of demand 60 Shortwave broadcasting AM broadcasting is also allowed in the shortwave bands by legacy radio stations Since radio waves in these bands can travel intercontinental distances by reflecting off the ionosphere using skywave or skip propagation shortwave is used by international stations broadcasting to other countries 60 61 nbsp FM broadcast transmitter of radio station KWNR Las Vegas with a power of 35 kW on 95 5 MHz nbsp FM broadcasting antenna nbsp AM FM boombox radio with FM whip antenna FM frequency modulation in FM the frequency of the radio carrier signal is varied slightly by the audio signal FM broadcasting is permitted in the FM broadcast bands between about 65 and 108 MHz in the very high frequency VHF range Radio waves in this band travel by line of sight so FM reception is limited by the visual horizon to about 30 40 mi 48 64 km and can be blocked by hills However it is less susceptible to interference from radio noise RFI sferics static and has higher fidelity better frequency response and less audio distortion than AM In the US radiated power ERP of FM stations varies from 6 100 kW 62 Digital radio involves a variety of standards and technologies for broadcasting digital radio signals over the air Some systems such as HD Radio and DRM operate in the same wavebands as analog broadcasts either as a replacement for analog stations or as a complementary service Others such as DAB DAB and ISDB Tsb operate in wavebands traditionally used for television or satellite services 63 nbsp Roberts radio for DABDigital Audio Broadcasting DAB debuted in some countries in 1998 It transmits audio as a digital signal rather than an analog signal as AM and FM do 64 DAB has the potential to provide higher quality sound than FM although many stations do not choose to transmit at such high quality has greater immunity to radio noise and interference makes better use of scarce radio spectrum bandwidth and provides advanced user features such as electronic program guides Its disadvantage is that it is incompatible with previous radios so that a new DAB receiver must be purchased 65 Several nations have set dates to switch off analog FM networks in favor of DAB DAB notably Norway in 2017 66 and Switzerland in 2024 67 A single DAB station transmits a 1 500 kHz bandwidth signal that carries from 9 12 channels of digital audio modulated by OFDM from which the listener can choose Broadcasters can transmit a channel at a range of different bit rates so different channels can have different audio quality In different countries DAB stations broadcast in either Band III 174 240 MHz or L band 1 452 1 492 GHz in the UHF range so like FM reception is limited by the visual horizon to about 40 miles 64 km 68 65 dd HD Radio is an alternative digital radio standard widely implemented in North America 69 An in band on channel technology HD Radio broadcasts a digital signal in a subcarrier of a station s analog FM or AM signal Stations are able to multicast more than one audio signal in the subcarrier supporting the transmission of multiple audio services at varying bitrates 70 The digital signal is transmitted using OFDM with the HDC High Definition Coding proprietary audio compression format HDC is based on but not compatible with the MPEG 4 standard HE AAC 71 It uses a modified discrete cosine transform MDCT audio data compression algorithm 72 Digital Radio Mondiale DRM is a competing digital terrestrial radio standard developed mainly by broadcasters as a higher spectral efficiency replacement for legacy AM and FM broadcasting Mondiale means worldwide in French and Italian DRM was developed in 2001 and is currently supported by 23 countries and adopted by some European and Eastern broadcasters beginning in 2003 The DRM30 mode uses the commercial broadcast bands below 30 MHz and is intended as a replacement for standard AM broadcast on the longwave mediumwave and shortwave bands The DRM mode uses VHF frequencies centered around the FM broadcast band and is intended as a replacement for FM broadcasting It is incompatible with existing radio receivers so it requires listeners to purchase a new DRM receiver The modulation used is a form of OFDM called COFDM in which up to 4 carriers are transmitted on a channel formerly occupied by a single AM or FM signal modulated by quadrature amplitude modulation QAM 73 61 The DRM system is designed to be as compatible as possible with existing AM and FM radio transmitters so that much of the equipment in existing radio stations can continue in use augmented with DRM modulation equipment 73 61 dd nbsp Volkswagen s RNS 510 receiver supports Sirius Satellite Radio Satellite radio is a subscription radio service that broadcasts CD quality digital audio direct to subscribers receivers using a microwave downlink signal from a direct broadcast communication satellite in geostationary orbit 22 000 miles 35 000 km above the Earth It is mostly intended for radios in vehicles Satellite radio uses the 2 3 GHz S band in North America in other parts of the world it uses the 1 4 GHz L band allocated for DAB 74 75 Video Television broadcasting edit Main article Television broadcasting Television broadcasting is the transmission of moving images by radio which consist of sequences of still images which are displayed on a screen on a television receiver a television or TV along with a synchronized audio sound channel Television video signals occupy a wider bandwidth than broadcast radio audio signals Analog television the original television technology required 6 MHz so the television frequency bands are divided into 6 MHz channels now called RF channels 76 The current television standard introduced beginning in 2006 is a digital format called high definition television HDTV which transmits pictures at higher resolution typically 1080 pixels high by 1920 pixels wide at a rate of 25 or 30 frames per second Digital television DTV transmission systems which replaced older analog television in a transition beginning in 2006 use image compression and high efficiency digital modulation such as OFDM and 8VSB to transmit HDTV video within a smaller bandwidth than the old analog channels saving scarce radio spectrum space Therefore each of the 6 MHz analog RF channels now carries up to 7 DTV channels these are called virtual channels Digital television receivers have different behavior in the presence of poor reception or noise than analog television called the digital cliff effect Unlike analog television in which increasingly poor reception causes the picture quality to gradually degrade in digital television picture quality is not affected by poor reception until at a certain point the receiver stops working and the screen goes black 77 78 nbsp Television studio control room Celebro Studios London nbsp A television broadcasting antenna nbsp A modern flatscreen television receiver Terrestrial television over the air OTA television or broadcast television the oldest television technology is the transmission of television signals from land based television stations to television receivers called televisions or TVs in viewer s homes Terrestrial television broadcasting uses the bands 41 88 MHz VHF low band or Band I carrying RF channels 1 6 174 240 MHz VHF high band or Band III carrying RF channels 7 13 and 470 614 MHz UHF Band IV and Band V carrying RF channels 14 and up 79 The exact frequency boundaries vary in different countries 80 Propagation is by line of sight so reception is limited by the visual horizon 81 In the US the effective radiated power ERP of television transmitters is regulated according to height above average terrain 82 Viewers closer to the television transmitter can use a simple rabbit ears dipole antenna on top of the TV but viewers in fringe reception areas typically require an outdoor antenna mounted on the roof to get adequate reception 81 nbsp nbsp left DISH Network s Super Dish 121 mounted on a rooftop right A residential tower block with TV satellite dishes used by various users Satellite television a set top box which receives subscription direct broadcast satellite television and displays it on an ordinary television A direct broadcast satellite in geostationary orbit 22 200 miles 35 700 km above the Earth s equator transmits many channels up to 900 modulated on a 12 2 to 12 7 GHz Ku band microwave downlink signal to a rooftop satellite dish antenna on the subscriber s residence The microwave signal is converted to a lower intermediate frequency at the dish and conducted into the building by a coaxial cable to a set top box connected to the subscriber s TV where it is demodulated and displayed The subscriber pays a monthly fee 83 84 Time edit Government standard frequency and time signal services operate time radio stations which continuously broadcast extremely accurate time signals produced by atomic clocks as a reference to synchronize other clocks 85 Examples are BPC DCF77 JJY MSF RTZ TDF WWV and YVTO 86 One use is in radio clocks and watches which include an automated receiver that periodically usually weekly receives and decodes the time signal and resets the watch s internal quartz clock to the correct time thus allowing a small watch or desk clock to have the same accuracy as an atomic clock Government time stations are declining in number because GPS satellites and the Internet Network Time Protocol NTP provide equally accurate time standards 87 Two way voice communication edit Main article Two way radio nbsp nbsp left Cellphones typical for Japan of the early 21st century right Cellular phone tower shared by antennas belonging to 3 different networks A two way radio is an audio transceiver a receiver and transmitter in the same device used for bidirectional person to person voice communication with other users with similar radios An older term for this mode of communication is radiotelephony The radio link may be half duplex as in a walkie talkie using a single radio channel in which only one radio can transmit at a time so different users take turns talking pressing a push to talk button on their radio which switches off the receiver and switches on the transmitter Or the radio link may be full duplex a bidirectional link using two radio channels so both people can talk at the same time as in a cell phone 88 Cell phone a portable wireless telephone that is connected to the telephone network by radio signals exchanged with a local antenna at a cellular base station cell tower 89 The service area covered by the provider is divided into small geographical areas called cells each served by a separate base station antenna and multichannel transceiver All the cell phones in a cell communicate with this antenna on separate frequency channels assigned from a common pool of frequencies The purpose of cellular organization is to conserve radio bandwidth by frequency reuse Low power transmitters are used so the radio waves used in a cell do not travel far beyond the cell allowing the same frequencies to be reused in geographically separated cells When a user carrying a cellphone crosses from one cell to another his phone is automatically handed off seamlessly to the new antenna and assigned new frequencies Cellphones have a highly automated full duplex digital transceiver using OFDM modulation using two digital radio channels each carrying one direction of the bidirectional conversation as well as a control channel that handles dialing calls and handing off the phone to another cell tower Older 2G 3G and 4G networks use frequencies in the UHF and low microwave range between 700 MHz and 3 GHz The cell phone transmitter adjusts its power output to use the minimum power necessary to communicate with the cell tower 0 6 W when near the tower up to 3 W when farther away Cell tower channel transmitter power is 50 W Current generation phones called smartphones have many functions besides making telephone calls and therefore have several other radio transmitters and receivers that connect them with other networks usually a Wi Fi modem a Bluetooth modem and a GPS receiver 90 91 92 nbsp nbsp left 5G millimeter wave antenna Germany right Polish 5G smartphones 5G cellular network next generation cellular networks which began deployment in 2019 Their major advantage is much higher data rates than previous cellular networks up to 10 Gbps 100 times faster than the previous cellular technology 4G LTE The higher data rates are achieved partly by using higher frequency radio waves in the higher microwave band 3 6 GHz and millimeter wave band around 28 and 39 GHz Since these frequencies have a shorter range than previous cellphone bands the cells will be smaller than the cells in previous cellular networks which could be many miles across Millimeter wave cells will only be a few blocks long and instead of a cell base station and antenna tower they will have many small antennas attached to utility poles and buildings 93 94 nbsp Satellite phones showing the large antennas needed to communicate with the satelliteSatellite phone satphone a portable wireless telephone similar to a cell phone connected to the telephone network through a radio link to an orbiting communications satellite instead of through cell towers They are more expensive than cell phones but their advantage is that unlike a cell phone which is limited to areas covered by cell towers satphones can be used over most or all of the geographical area of the Earth In order for the phone to communicate with a satellite using a small omnidirectional antenna first generation systems use satellites in low Earth orbit about 400 700 miles 640 1 100 km above the surface With an orbital period of about 100 minutes a satellite can only be in view of a phone for about 4 15 minutes so the call is handed off to another satellite when one passes beyond the local horizon Therefore large numbers of satellites about 40 to 70 are required to ensure that at least one satellite is in view continuously from each point on Earth Other satphone systems use satellites in geostationary orbit in which only a few satellites are needed but these cannot be used at high latitudes because of terrestrial interference 95 96 Cordless phone a landline telephone in which the handset is portable and communicates with the rest of the phone by a short range full duplex radio link instead of being attached by a cord Both the handset and the base station have low power radio transceivers that handle the short range bidirectional radio link 97 As of 2022 update cordless phones in most nations use the DECT transmission standard 98 nbsp Motorola SCR 536 from WW2 the first walkie talkieLand mobile radio system short range mobile or portable half duplex radio transceivers operating in the VHF or UHF band that can be used without a license They are often installed in vehicles with the mobile units communicating with a dispatcher at a fixed base station Special systems with reserved frequencies are used by first responder services police fire ambulance and emergency services and other government services Other systems are made for use by commercial firms such as taxi and delivery services VHF systems use channels in the range 30 50 MHz and 150 172 MHz UHF systems use the 450 470 MHz band and in some areas the 470 512 MHz range In general VHF systems have a longer range than UHF but require longer antennas AM or FM modulation is mainly used but digital systems such as DMR are being introduced The radiated power is typically limited to 4 watts 89 These systems have a fairly limited range usually 3 to 20 miles 4 8 to 32 km depending on terrain Repeaters installed on tall buildings hills or mountain peaks are often used to increase the range when it is desired to cover a larger area than line of sight Examples of land mobile systems are CB FRS GMRS and MURS Modern digital systems called trunked radio systems have a digital channel management system using a control channel that automatically assigns frequency channels to user groups 99 nbsp Firefighter using modern walkie talkieWalkie talkie a battery powered portable handheld half duplex two way radio used in land mobile radio systems 100 Airband Half duplex radio system used by aircraft pilots to talk to other aircraft and ground based air traffic controllers This vital system is the main communication channel for air traffic control For most communication in overland flights in air corridors a VHF AM system using channels between 108 and 137 MHz in the VHF band is used This system has a typical transmission range of 200 miles 320 km for aircraft flying at cruising altitude 101 102 For flights in more remote areas such as transoceanic airline flights aircraft use the HF band or channels on the Inmarsat or Iridium satphone satellites 103 Military aircraft also use a dedicated UHF AM band from 225 0 to 399 95 MHz 104 nbsp VHF marine radio on a shipMarine radio medium range transceivers on ships used for ship to ship ship to air and ship to shore communication with harbormasters They use FM channels between 156 and 174 MHz in the VHF band with up to 25 watts power giving them a range of about 60 miles 97 km Some channels are half duplex and some are full duplex to be compatible with the telephone network to allow users to make telephone calls through a marine operator 105 Amateur radio long range half duplex two way radio used by hobbyists for non commercial purposes recreational radio contacts with other amateurs volunteer emergency communication during disasters contests and experimentation Radio amateurs must hold an amateur radio license and are given a unique callsign that must be used as an identifier in transmissions Amateur radio is restricted to small frequency bands the amateur radio bands spaced throughout the radio spectrum starting at 136 kHz Within these bands amateurs are allowed the freedom to transmit on any frequency using a wide variety of voice modulation methods along with other forms of communication such as slow scan television SSTV and radioteletype RTTY Additionally amateurs are among the only radio operators still using Morse code radiotelegraphy 106 One way voice communication edit One way unidirectional radio transmission is called simplex Baby monitor a crib side appliance for parents of infants that transmits the baby s sounds to a receiver carried by the parent so they can monitor the baby while they are in other parts of the house 107 The wavebands used vary by region but analog baby monitors generally transmit with low power in the 16 9 3 49 9 or 900 MHz wavebands and digital systems in the 2 4 GHz waveband 108 Many baby monitors have duplex channels so the parent can talk to the baby and cameras to show video of the baby 109 Wireless microphone a battery powered microphone with a short range transmitter that is handheld or worn on a person s body which transmits its sound by radio to a nearby receiver unit connected to a sound system Wireless microphones are used by public speakers performers and television personalities so they can move freely without trailing a microphone cord Traditionally analog models transmit in FM on unused portions of the television broadcast frequencies in the VHF and UHF bands Some models transmit on two frequency channels for diversity reception to prevent nulls from interrupting transmission as the performer moves around 110 Some models use digital modulation to prevent unauthorized reception by scanner radio receivers these operate in the 900 MHz 2 4 GHz or 6 GHz ISM bands 111 European standards also support wireless multichannel audio systems WMAS that can better support the use of large numbers of wireless microphones at a single event or venue As of 2021 update U S regulators were considering adopting rules for WMAS 112 Data communication edit Further information Data transmission Telemetry and Wireless Wireless networking automated radio links which transmit digital data between computers and other wireless devices using radio waves linking the devices together transparently in a computer network Computer networks can transmit any form of data in addition to email and web pages they also carry phone calls VoIP audio and video content called streaming media Security is more of an issue for wireless networks than for wired networks since anyone nearby with a wireless modem can access the signal and attempt to log in The radio signals of wireless networks are encrypted using WPA 113 nbsp A laptop with Wi Fi module and a typical home wireless router on the right connecting it to the Internet The laptop shows its own photoWireless LAN wireless local area network or Wi Fi based on the IEEE 802 11 standards these are the most widely used computer networks used to implement local area networks without cables linking computers laptops cell phones video game consoles smart TVs and printers in a home or office together and to a wireless router connecting them to the Internet with a wire or cable connection Wireless routers in public places like libraries hotels and coffee shops create wireless access points hotspots to allow the public to access the Internet with portable devices like smartphones tablets or laptops Each device exchanges data using a wireless modem wireless network interface controller an automated microwave transmitter and receiver with an omnidirectional antenna that works in the background exchanging data packets with the router Wi Fi uses channels in the 2 4 GHz and 5 GHz ISM bands with OFDM orthogonal frequency division multiplexing modulation to transmit data at high rates The transmitters in Wi Fi modems are limited to a radiated power of 200 mW to 1 watt depending on country They have a maximum indoor range of about 150 ft 50 m on 2 4 GHz and 50 ft 20 m on 5 GHz 114 nbsp Neighborhood wireless WAN router on telephone poleWireless WAN wireless wide area network WWAN a variety of technologies that provide wireless internet access over a wider area than Wi Fi networks do from an office building to a campus to a neighborhood or to an entire city The most common technologies used are cellular modems that exchange computer data by radio with cell towers satellite internet access and lower frequencies in the UHF band which have a longer range than Wi Fi frequencies Since WWAN networks are much more expensive and complicated to administer than Wi Fi networks their use so far has generally been limited to private networks operated by large corporations 114 Bluetooth a very short range wireless interface on a portable wireless device used as a substitute for a wire or cable connection mainly to exchange files between portable devices and connect cellphones and music players with wireless headphones In the most widely used mode transmission power is limited to 1 milliwatt giving it a very short range of up to 10 m 30 feet The system uses frequency hopping spread spectrum transmission in which successive data packets are transmitted in a pseudorandom order on one of 79 1 MHz Bluetooth channels between 2 4 and 2 83 GHz in the ISM band This allows Bluetooth networks to operate in the presence of noise other wireless devices and other Bluetooth networks using the same frequencies since the chance of another device attempting to transmit on the same frequency at the same time as the Bluetooth modem is low In the case of such a collision the Bluetooth modem just retransmits the data packet on another frequency 115 Packet radio a long distance peer to peer wireless ad hoc network in which data packets are exchanged between computer controlled radio modems transmitter receivers called nodes which may be separated by miles and maybe mobile Each node only communicates with neighboring nodes so packets of data are passed from node to node until they reach their destination using the X 25 network protocol Packet radio systems are used to a limited degree by commercial telecommunications companies and by the amateur radio community 116 Text messaging texting this is a service on cell phones allowing a user to type a short alphanumeric message and send it to another phone number and the text is displayed on the recipient s phone screen It is based on the Short Message Service SMS which transmits using spare bandwidth on the control radio channel used by cell phones to handle background functions like dialing and cell handoffs Due to technical limitations of the channel text messages are limited to 160 alphanumeric characters 117 nbsp Parabolic antennas of microwave relay links on tower in AustraliaMicrowave relay a long distance high bandwidth point to point digital data transmission link consisting of a microwave transmitter connected to a dish antenna that transmits a beam of microwaves to another dish antenna and receiver Since the antennas must be in line of sight distances are limited by the visual horizon to 30 40 miles 48 64 km Microwave links are used for private business data wide area computer networks WANs and by telephone companies to transmit long distance phone calls and television signals between cities 118 119 Telemetry automated one way simplex transmission of measurements and operation data from a remote process or device to a receiver for monitoring Telemetry is used for in flight monitoring of missiles drones satellites and weather balloon radiosondes sending scientific data back to Earth from interplanetary spacecraft communicating with electronic biomedical sensors implanted in the human body and well logging Multiple channels of data are often transmitted using frequency division multiplexing or time division multiplexing 120 Telemetry is starting to be used in consumer applications such as Automated meter reading electric power meters water meters and gas meters that when triggered by an interrogation signal transmit their readings by radio to a utility reader vehicle at the curb to eliminate the need for an employee to go on the customer s property to manually read the meter 121 Electronic toll collection on toll roads an alternative to manual collection of tolls at a toll booth in which a transponder in a vehicle when triggered by a roadside transmitter transmits a signal to a roadside receiver to register the vehicle s use of the road enabling the owner to be billed for the toll 122 nbsp RFID tag from a DVDRadio Frequency Identification RFID identification tags containing a tiny radio transponder receiver and transmitter which are attached to merchandise When it receives an interrogation pulse of radio waves from a nearby reader unit the tag transmits back an ID number which can be used to inventory goods Passive tags the most common type have a chip powered by the radio energy received from the reader rectified by a diode and can be as small as a grain of rice They are incorporated in products clothes railroad cars library books airline baggage tags and are implanted under the skin in pets and livestock microchip implant and even people Privacy concerns have been addressed with tags that use encrypted signals and authenticate the reader before responding Passive tags use 125 134 kHz 13 900 MHz and 2 4 and 5 GHz ISM bands and have a short range Active tags powered by a battery are larger but can transmit a stronger signal giving them a range of hundreds of meters 123 Submarine communication When submerged submarines are cut off from all ordinary radio communication with their military command authorities by the conductive seawater However radio waves of low enough frequencies in the VLF 30 to 3 kHz and ELF below 3 kHz bands are able to penetrate seawater Navies operate large shore transmitting stations with power output in the megawatt range to transmit encrypted messages to their submarines in the world s oceans Due to the small bandwidth these systems cannot transmit voice only text messages at a slow data rate The communication channel is one way since the long antennas needed to transmit VLF or ELF waves cannot fit on a submarine VLF transmitters use miles long wire antennas like umbrella antennas A few nations use ELF transmitters operating around 80 Hz which can communicate with submarines at lower depths These use even larger antennas called ground dipoles consisting of two ground Earth connections 23 60 km 14 37 mi apart linked by overhead transmission lines to a power plant transmitter 124 125 Space communication edit nbsp Satellite Communications Center Dubna in Russia 126 This is radio communication between a spacecraft and an Earth based ground station or another spacecraft Communication with spacecraft involves the longest transmission distances of any radio links up to billions of kilometers for interplanetary spacecraft In order to receive the weak signals from distant spacecraft satellite ground stations use large parabolic dish antennas up to 25 metres 82 ft in diameter and extremely sensitive receivers High frequencies in the microwave band are used since microwaves pass through the ionosphere without refraction and at microwave frequencies the high gain antennas needed to focus the radio energy into a narrow beam pointed at the receiver are small and take up a minimum of space in a satellite Portions of the UHF L C S ku and ka band are allocated for space communication A radio link that transmits data from the Earth s surface to a spacecraft is called an uplink while a link that transmits data from the spacecraft to the ground is called a downlink 127 nbsp Communications satellite belonging to AzerbaijanCommunication satellite an artificial satellite used as a telecommunications relay to transmit data between widely separated points on Earth These are used because the microwaves used for telecommunications travel by line of sight and so cannot propagate around the curve of the Earth As of 1 January 2021 update there were 2 224 communications satellites in Earth orbit 128 Most are in geostationary orbit 22 200 miles 35 700 km above the equator so that the satellite appears stationary at the same point in the sky so the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track it In a satellite ground station a microwave transmitter and large satellite dish antenna transmit a microwave uplink beam to the satellite The uplink signal carries many channels of telecommunications traffic such as long distance telephone calls television programs and internet signals using a technique called frequency division multiplexing FDM On the satellite a transponder receives the signal translates it to a different downlink frequency to avoid interfering with the uplink signal and retransmits it down to another ground station which may be widely separated from the first There the downlink signal is demodulated and the telecommunications traffic it carries is sent to its local destinations through landlines Communication satellites typically have several dozen transponders on different frequencies which are leased by different users 129 Direct broadcast satellite a geostationary communication satellite that transmits retail programming directly to receivers in subscriber s homes and vehicles on Earth in satellite radio and TV systems It uses a higher transmitter power than other communication satellites to allow the signal to be received by consumers with a small unobtrusive antenna For example satellite television uses downlink frequencies from 12 2 to 12 7 GHz in the ku band transmitted at 100 to 250 watts which can be received by relatively small 43 80 cm 17 31 in satellite dishes mounted on the outside of buildings 130 Radar edit Main article Radar nbsp Military air traffic controller on US Navy aircraft carrier monitors aircraft on radar screenRadar is a radiolocation method used to locate and track aircraft spacecraft missiles ships vehicles and also to map weather patterns and terrain A radar set consists of a transmitter and receiver 131 132 The transmitter emits a narrow beam of radio waves which is swept around the surrounding space When the beam strikes a target object radio waves are reflected back to the receiver The direction of the beam reveals the object s location Since radio waves travel at a constant speed close to the speed of light by measuring the brief time delay between the outgoing pulse and the received echo the range to the target can be calculated The targets are often displayed graphically on a map display called a radar screen Doppler radar can measure a moving object s velocity by measuring the change in frequency of the return radio waves due to the Doppler effect 133 Radar sets mainly use high frequencies in the microwave bands because these frequencies create strong reflections from objects the size of vehicles and can be focused into narrow beams with compact antennas 132 Parabolic dish antennas are widely used In most radars the transmitting antenna also serves as the receiving antenna this is called a monostatic radar A radar which uses separate transmitting and receiving antennas is called a bistatic radar 134 nbsp ASR 8 airport surveillance radar antenna It rotates once every 4 8 seconds The rectangular antenna on top is the secondary radar Airport surveillance radar In aviation radar is the main tool of air traffic control A rotating dish antenna sweeps a vertical fan shaped beam of microwaves around the airspace and the radar set shows the location of aircraft as blips of light on a display called a radar screen Airport radar operates at 2 7 2 9 GHz in the microwave S band In large airports the radar image is displayed on multiple screens in an operations room called the TRACON Terminal Radar Approach Control where air traffic controllers direct the aircraft by radio to maintain safe aircraft separation 135 Secondary surveillance radar Aircraft carry radar transponders transceivers which when triggered by the incoming radar signal transmit a return microwave signal This causes the aircraft to show up more strongly on the radar screen The radar which triggers the transponder and receives the return beam usually mounted on top of the primary radar dish is called the secondary surveillance radar Since radar cannot measure an aircraft s altitude with any accuracy the transponder also transmits back the aircraft s altitude measured by its altimeter and an ID number identifying the aircraft which is displayed on the radar screen 136 Electronic countermeasures ECM Military defensive electronic systems designed to degrade enemy radar effectiveness or deceive it with false information to prevent enemies from locating local forces It often consists of powerful microwave transmitters that can mimic enemy radar signals to create false target indications on the enemy radar screens 137 nbsp Rotating marine radar antenna on a shipMarine radar an S or X band radar on ships used to detect nearby ships and obstructions like bridges 138 A rotating antenna sweeps a vertical fan shaped beam of microwaves around the water surface surrounding the craft out to the horizon Weather radar A Doppler radar which maps weather precipitation intensities and wind speeds with the echoes returned from raindrops and their radial velocity by their Doppler shift 139 Phased array radar a radar set that uses a phased array a computer controlled antenna that can steer the radar beam quickly to point in different directions without moving the antenna Phased array radars were developed by the military to track fast moving missiles and aircraft They are widely used in military equipment and are now spreading to civilian applications 140 Synthetic aperture radar SAR a specialized airborne radar set that produces a high resolution map of ground terrain The radar is mounted on an aircraft or spacecraft and the radar antenna radiates a beam of radio waves sideways at right angles to the direction of motion toward the ground In processing the return radar signal the motion of the vehicle is used to simulate a large antenna giving the radar a higher resolution 141 Ground penetrating radar a specialized radar instrument that is rolled along the ground surface in a cart and transmits a beam of radio waves into the ground producing an image of subsurface objects Frequencies from 100 MHz to a few GHz are used Since radio waves cannot penetrate very far into earth the depth of GPR is limited to about 50 feet 142 Collision avoidance system a short range radar or LIDAR system on an automobile or vehicle that detects if the vehicle is about to collide with an object and applies the brakes to prevent the collision 143 Radar fuze a detonator for an aerial bomb which uses a radar altimeter to measure the height of the bomb above the ground as it falls and detonates it at a certain altitude 144 Radiolocation edit Main article Radiolocation Radiolocation is a generic term covering a variety of techniques that use radio waves to find the location of objects or for navigation 145 nbsp An early iPhone with its GPS navigation app in use Global Navigation Satellite System GNSS or satnav system A system of satellites which allows geographical location on Earth latitude longitude and altitude elevation to be determined to high precision within a few metres by small portable navigation instruments by timing the arrival of radio signals from the satellites These are the most widely used navigation systems today The main satellite navigation systems are the US Global Positioning System GPS Russia s GLONASS China s BeiDou Navigation Satellite System BDS and the European Union s Galileo 146 nbsp A personal navigation assistant by Garmin which uses GPS to give driving directions to a destination Global Positioning System GPS The most widely used satellite navigation system maintained by the US Air Force which uses a constellation of 31 satellites in low Earth orbit The orbits of the satellites are distributed so at any time at least four satellites are above the horizon over each point on Earth Each satellite has an onboard atomic clock and transmits a continuous radio signal containing a precise time signal as well as its current position Two frequencies are used 1 2276 and 1 57542 GHz Since the velocity of radio waves is virtually constant the delay of the radio signal from a satellite is proportional to the distance of the receiver from the satellite By receiving the signals from at least four satellites a GPS receiver can calculate its position on Earth by comparing the arrival time of the radio signals Since each satellite s position is known precisely at any given time from the delay the position of the receiver can be calculated by a microprocessor in the receiver The position can be displayed as latitude and longitude or as a marker on an electronic map GPS receivers are incorporated in almost all cellphones and in vehicles such as automobiles aircraft and ships and are used to guide drones missiles cruise missiles and even artillery shells to their target and handheld GPS receivers are produced for hikers and the military 147 148 Radio beacon a fixed location terrestrial radio transmitter which transmits a continuous radio signal used by aircraft and ships for navigation The locations of beacons are plotted on navigational maps used by aircraft and ships 149 VHF omnidirectional range VOR a worldwide aircraft radio navigation system consisting of fixed ground radio beacons transmitting between 108 00 and 117 95 MHz in the very high frequency VHF band An automated navigational instrument on the aircraft displays a bearing to a nearby VOR transmitter A VOR beacon transmits two signals simultaneously on different frequencies A directional antenna transmits a beam of radio waves that rotates like a lighthouse at a fixed rate 30 times per second When the directional beam is facing north an omnidirectional antenna transmits a pulse By measuring the difference in phase of these two signals an aircraft can determine its bearing or radial from the station accurately By taking a bearing on two VOR beacons an aircraft can determine its position called a fix to an accuracy of about 90 metres 300 ft Most VOR beacons also have a distance measuring capability called distance measuring equipment DME these are called VOR DME s The aircraft transmits a radio signal to the VOR DME beacon and a transponder transmits a return signal From the propagation delay between the transmitted and received signal the aircraft can calculate its distance from the beacon This allows an aircraft to determine its location fix from only one VOR beacon Since line of sight VHF frequencies are used VOR beacons have a range of about 200 miles for aircraft at cruising altitude TACAN is a similar military radio beacon system which transmits in 962 1213 MHz and a combined VOR and TACAN beacon is called a VORTAC The number of VOR beacons is declining as aviation switches to the RNAV system that relies on Global Positioning System satellite navigation 150 151 Non directional beacon NDB Legacy fixed radio beacons used before the VOR system that transmit a simple signal in all directions for aircraft or ships to use for radio direction finding Aircraft use automatic direction finder ADF receivers which use a directional antenna to determine the bearing to the beacon By taking bearings on two beacons they can determine their position NDBs use frequencies between 190 and 1750 kHz in the LF and MF bands which propagate beyond the horizon as ground waves or skywaves much farther than VOR beacons They transmit a callsign consisting of one to 3 Morse code letters as an identifier 152 nbsp EPIRB emergency locator beacon on a shipEmergency locator beacon a portable battery powered radio transmitter used in emergencies to locate airplanes vessels and persons in distress and in need of immediate rescue Various types of emergency locator beacons are carried by aircraft ships vehicles hikers and cross country skiers In the event of an emergency such as the aircraft crashing the ship sinking or a hiker becoming lost the transmitter is deployed and begins to transmit a continuous radio signal which is used by search and rescue teams to quickly find the emergency and render aid The latest generation Emergency Position Indicating Rescue Beacons EPIRBs contain a GPS receiver and broadcast to rescue teams their exact location within 20 meters 153 Cospas Sarsat an international humanitarian consortium of governmental and private agencies which acts as a dispatcher for search and rescue operations It operates a network of about 47 satellites carrying radio receivers which detect distress signals from emergency locator beacons anywhere on Earth transmitting on the international Cospas distress frequency of 406 MHz The satellites calculate the geographic location of the beacon within 2 km by measuring the Doppler frequency shift of the radio waves due to the relative motion of the transmitter and the satellite and quickly transmit the information to the appropriate local first responder organizations which perform the search and rescue 154 155 nbsp Wildlife officer tracking radio tagged mountain lionRadio direction finding RDF this is a general technique used since the early 1900s of using specialized radio receivers with directional antennas RDF receivers to determine the exact bearing of a radio signal to determine the location of the transmitter The location of a terrestrial transmitter can be determined by simple triangulation from bearings taken by two RDF stations separated geographically as the point where the two bearing lines cross this is called a fix Military forces use RDF to locate enemy forces by their tactical radio transmissions counterintelligence services use it to locate clandestine transmitters used by espionage agents and governments use it to locate unlicensed transmitters or interference sources Older RDF receivers used rotatable loop antennas the antenna is rotated until the radio signal strength is weakest indicating the transmitter is in one of the antenna s two nulls The nulls are used since they are sharper than the antenna s lobes maxima More modern receivers use phased array antennas which have a much greater angular resolution 156 157 Animal migration tracking a widely used technique in wildlife biology conservation biology and wildlife management in which small battery powered radio transmitters are attached to wild animals so their movements can be tracked with a directional RDF receiver Sometimes the transmitter is implanted in the animal The VHF band is typically used since antennas in this band are fairly compact The receiver has a directional antenna typically a small Yagi which is rotated until the received signal is strongest at this point the antenna is pointing in the direction of the animal Sophisticated systems used in recent years use satellites to track the animal or geolocation tags with GPS receivers which record and transmit a log of the animal s location 158 Remote control edit Main article Radio control nbsp US Air Force MQ 1 Predator drone flown remotely by a pilot on the groundRadio remote control is the use of electronic control signals sent by radio waves from a transmitter to control the actions of a device at a remote location Remote control systems may also include telemetry channels in the other direction used to transmit real time information on the state of the device back to the control station Uncrewed spacecraft are an example of remote controlled machines controlled by commands transmitted by satellite ground stations Most handheld remote controls used to control consumer electronics products like televisions or DVD players actually operate by infrared light rather than radio waves so are not examples of radio remote control A security concern with remote control systems is spoofing in which an unauthorized person transmits an imitation of the control signal to take control of the device 159 Examples of radio remote control Unmanned aerial vehicle UAV drone A drone is an aircraft without an onboard pilot flown by remote control by a pilot in another location usually in a piloting station on the ground They are used by the military for reconnaissance and ground attack and more recently by the civilian world for news reporting and aerial photography The pilot uses aircraft controls like a joystick or steering wheel which create control signals which are transmitted to the drone by radio to control the flight surfaces and engine A telemetry system transmits back a video image from a camera in the drone to allow the pilot to see where the aircraft is going and data from a GPS receiver giving the real time position of the aircraft UAVs have sophisticated onboard automatic pilot systems that maintain stable flight and only require manual control to change directions 160 nbsp Remote keyless entry fob for a carKeyless entry system a short range handheld battery powered key fob transmitter included with most modern cars which can lock and unlock the doors of a vehicle from outside eliminating the need to use a key When a button is pressed the transmitter sends a coded radio signal to a receiver in the vehicle operating the locks The fob must be close to the vehicle typically within 5 to 20 meters North America and Japan use a frequency of 315 MHz while Europe uses 433 92 and 868 MHz Some models can also remotely start the engine to warm up the car A security concern with all keyless entry systems is a replay attack in which a thief uses a special receiver code grabber to record the radio signal during opening which can later be replayed to open the door To prevent this keyless systems use a rolling code system in which a pseudorandom number generator in the remote control generates a different random key each time it is used To prevent thieves from simulating the pseudorandom generator to calculate the next key the radio signal is also encrypted 161 Garage door opener a short range handheld transmitter which can open or close a building s electrically operated garage door from outside so the owner can open the door upon arrival and close it after departure When a button is pressed the control transmits a coded FSK radio signal to a receiver in the opener raising or lowering the door Modern openers use 310 315 or 390 MHz To prevent a thief using a replay attack modern openers use a rolling code system 162 163 nbsp Quadcopter a popular remote controlled toyRadio controlled models a popular hobby is playing with radio controlled model boats cars airplanes and helicopters quadcopters which are controlled by radio signals from a handheld console with a joystick Most recent transmitters use the 2 4 GHz ISM band with multiple control channels modulated with PWM PCM or FSK 164 Wireless doorbell A residential doorbell that uses wireless technology to eliminate the need to run wires through the building walls It consists of a doorbell button beside the door containing a small battery powered transmitter When the doorbell is pressed it sends a signal to a receiver inside the house with a speaker that sounds chimes to indicate someone is at the door They usually use the 2 4 GHz ISM band The frequency channel used can usually be changed by the owner in case another nearby doorbell is using the same channel 165 166 Jamming edit Radio jamming is the deliberate radiation of radio signals designed to interfere with the reception of other radio signals Jamming devices are called signal suppressors or interference generators or just jammers 167 During wartime militaries use jamming to interfere with enemies tactical radio communication Since radio waves can pass beyond national borders some totalitarian countries which practice censorship use jamming to prevent their citizens from listening to broadcasts from radio stations in other countries Jamming is usually accomplished by a powerful transmitter which generates noise on the same frequency as the target transmitter 168 169 US Federal law prohibits the nonmilitary operation or sale of any type of jamming devices including ones that interfere with GPS cellular Wi Fi and police radars 170 Scientific research edit Radio astronomy is the scientific study of radio waves emitted by astronomical objects Radio astronomers use radio telescopes large radio antennas and receivers to receive and study the radio waves from astronomical radio sources Since astronomical radio sources are so far away the radio waves from them are extremely weak requiring extremely sensitive receivers and radio telescopes are the most sensitive radio receivers in existence They use large parabolic dish antennas up to 500 meters 2 000 ft in diameter to collect enough radio wave energy to study The RF front end electronics of the receiver is often cooled by liquid nitrogen to reduce thermal noise Multiple antennas are often linked together in arrays which function as a single antenna to increase collecting power In Very Long Baseline Interferometry VLBI radio telescopes on different continents are linked which can achieve the resolution of an antenna thousands of miles in diameter 171 172 Remote sensing in radio remote sensing is the reception of electromagnetic waves radiated by natural objects or the atmosphere for scientific research All warm objects emit microwaves and the spectrum emitted can be used to determine temperature Microwave radiometers are used in meteorology and earth sciences to determine temperature of the atmosphere and earth surface as well as chemical reactions in the atmosphere 173 174 See also edit nbsp Radio portalOutline of radio Electromagnetic radiation and health List of radios List of specific models of radios Radio quiet zoneReferences edit Radio Oxford Living Dictionaries Oxford University Press 2019 Archived from the original on March 24 2019 Retrieved 26 February 2019 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to resist it Ajax Systems Blog Ajax Systems Retrieved 2020 01 18 Remedial Electronic Counter Countermeasures Techniques FM 24 33 Communications Techniques Electronic Counter Countermeasures Report Department of the Army July 1990 Varis Tapio 1970 The Control of Information by Jamming Radio Broadcasts Cooperation and Conflict 5 3 168 184 doi 10 1177 001083677000500303 ISSN 0010 8367 JSTOR 45083158 S2CID 145418504 Jammer Enforcement Federal Communications Commission 2011 03 03 Retrieved 2020 01 18 Yeap Kim Ho Hirasawa Kazuhiro 2020 Analyzing the Physics of Radio Telescopes and Radio Astronomy IG Global ISBN 978 1799823834 Retrieved 9 September 2022 Joardar Shubhendu Claycomb J R 2015 Radio Astronomy An Introduction Mercury Learning and Information ISBN 978 1937585624 Chapman Rick Gasparovic Richard 2022 Remote Sensing Physics An Introduction to Observing Earth from Space Wiley ISBN 978 1119669074 Retrieved 9 September 2022 Pampaloni Paulo Paloscia S 2000 Microwave Radiometry and Remote Sensing of the Earth s Surface and Atmosphere ISBN 9067643181 Retrieved 9 September 2022 General references editBasic Radio Principles and Technology Elsevier Science The Electronics of Radio Cambridge University Press Radio Systems Engineering Cambridge University Press Radio Electronic Transmission Fundamentals SciTech Publishing Analog Electronics Analog Circuitry Explained Elsevier ScienceExternal links edit nbsp Wikimedia Commons has media related to Radio category nbsp Wikiquote has quotations related to Radio Radio Merriam Webster com Dictionary Retrieved from https en wikipedia org w index php title Radio amp oldid 1186145371, wikipedia, wiki, book, books, library,

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