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

Radio is the technology of communicating using radio waves.[1][2][3] Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connected to an antenna which radiates the waves. They are received by another antenna connected to a radio receiver. In addition to communication, radio is used for radar, radio navigation, remote control, remote sensing, and other applications.

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 11 November 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 12 December 1901.[7] The first commercial radio broadcast was transmitted on 2 November 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.[8]

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.


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.[9][10]

Following Heinrich Hertz's discovery of the existence of radio waves in 1886, the term "Hertzian waves" was initially used for this radiation.[11] 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.[12][13]

The French physicist Édouard Branly, who in 1890 developed the radio wave detecting coherer, called it in French a radio-conducteur.[14][15] 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".[14][16]

The use of "radio" as a standalone word dates back to at least 30 December 1904, when instructions issued by the British Post Office for transmitting telegrams specified that "The word 'Radio'... is sent in the Service Instructions".[14][17] 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'".[14]

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 22 June 1907 Electrical World about the need for legal restrictions warned that "Radio chaos will certainly be the result until such stringent regulation is enforced".[18] 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.


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 11 November 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.[19] 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] Marconi and Karl Ferdinand Braun shared the 1909 Nobel Prize in Physics "for their contributions to the development of wireless telegraphy".[20]

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, 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 2 November 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.[8]


Radio waves are radiated by electric charges undergoing acceleration.[21][22] They are generated artificially by time varying electric currents, consisting of electrons flowing back and forth in a metal conductor called an antenna.[23][24]

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.[25][26][27]

Radio waves travel at the speed of light in vacuum.[28][29]

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.[24]

Radio communication

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.[24][30] 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:[31]

Many other types of modulation are also used. In some types, a carrier wave is not transmitted but just one or both modulation sidebands.[33]

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.[34] 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.[24][30] 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.[31]

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.[30] 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.[35]


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).[31][36] 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.[31][37]

The radio spectrum, the total range of radio frequencies that can be used for communication in a given area, is a limited resource.[36][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.[37]

A slow transition from analog to digital radio transmission technologies began in the late 1990s.[38][39] 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.[31]

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.[37]

ITU frequency bands

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][40] Each of these bands has a traditional name:[41]

Band name Abbreviation Frequency Wavelength
low frequency
ELF 3–30 Hz 100,000–
10,000 km
low frequency
SLF 30–300 Hz 10,000 –
1,000 km
low frequency
ULF 300–
3,000 Hz
100 km
low frequency
VLF 3–30 kHz 100–10 km
LF 30–300 kHz 10–1 km
MF 300–
3,000 kHz
100 m
Band name Abbreviation Frequency Wavelength
HF 3–30 MHz 100–10 m
high frequency
VHF 30–300 MHz 10–1 m
high frequency
UHF 300–
3,000 MHz
100–10 cm
high frequency
SHF 3–30 GHz 10–1 cm
high frequency
EHF 30–300 GHz 10–1 mm
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.[42]

The term "tremendously low frequency" (TLF) has been used for wavelengths from 1–3 Hz (300,000–100,000 km),[43] but the term has not been defined by the ITU.[41]


Further 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.[36] 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).[44][45]

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.[36][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.[46] 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.[47]

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.[48]


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


Further information: Broadcasting

Broadcasting is the one-way transmission of information from a transmitter to receivers belonging to a public audience.[49] Since the radio waves become weaker with distance, a broadcasting station can only be received within a limited distance of its transmitter.[50] 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.[51]

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.[52][53]

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.[54][55]

Audio: Radio broadcasting

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.[56]

1100 W AM broadcasting transmitter
Mast radiator antenna of AM radio station
Panasonic AM radio from 1964
  • 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.[58][59]
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
"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.[62] 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.[63] Several nations have set dates to switch off analog FM networks in favor of DAB / DAB+, notably Norway in 2017[64] and Switzerland in 2024.[65]
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).[66][63]
  • 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).[71][59]
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.[71][59]
Volkswagen's RNS-510 receiver supports Sirius Satellite Radio.

Video: Television broadcasting

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".[74]

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.[75][76]

Television studio control room, Celebro Studios, London
A television broadcasting antenna
A modern flatscreen television receiver
(left) DISH Network's Super Dish 121 mounted on a rooftop. (right) A residential tower block with TV satellite dishes used by various users


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.[83] Examples are BPC, DCF77, JJY, MSF, RTZ, TDF, WWV, and YVTO.[84] 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.[85]

Two-way voice communication

Main article: Two-way radio

Cellphones typical of Japan in the early 21st century.
Modern smartphone
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.[86]

(left) 5G millimeter wave antenna, Germany (right) Polish 5G smartphones
Satellite phones, showing the large antennas needed to communicate with the satellite
Motorola SCR-536 from WW2, the first walkie-talkie
Firefighter using modern walkie-talkie
VHF marine radio on a ship

One-way voice communication

One way, unidirectional radio transmission is called simplex.

Data communication

Further information: Data transmission, Telemetry, and Wireless

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
Parabolic antennas of microwave relay links on tower in Australia
RFID tag from a DVD

Space communication

Satellite Communications Center Dubna in Russia[124]

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.[125]

Communications satellite belonging to Azerbaijan


Main article: Radar

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.[129][130] 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.[131]

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.[130] 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.[132]

ASR-8 airport surveillance radar antenna. It rotates once every 4.8 seconds. The rectangular antenna on top is the secondary radar.
Rotating marine radar antenna on a ship


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.[143]

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.
EPIRB emergency locator beacon on a ship
Wildlife officer tracking radio-tagged mountain lion

Remote control

Main article: Radio control

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.[157] Examples of radio remote control:

Remote keyless entry fob for a car
Quadcopter, a popular remote-controlled toy


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.[165]

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.[166][167]

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.[168]

Scientific research

See also


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  14. ^ a b c d https://earlyradiohistory.us/sec022.htm Thomas H. White, United States Early Radio History, Section 22
  15. ^ Collins, A. Frederick (10 May 1902). "The Genesis of Wireless Telegraphy". Electrical World and Engineer. p. 811.
  16. ^ "Wireless Telegraphy". The Practical Engineer. 25 February 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.
  17. ^ "Wireless Telegraphy", The Electrical Review (London), 20 January 1905, page 108, quoting from the British Post Office's 30 December 1904 Post Office Circular.
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General references