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transmission or reception of electromagnetic radiationelectromagnetic radiation,
energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an electric field.
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 in the radio frequencyradio frequency,
range of electromagnetic waves with a frequency or wavelength suitable for communication uses. Some of these waves serve as carriers of the lower-frequency audio waves; others are modulated by video or digital information.
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 range. The term is commonly applied also to the equipment used, especially to the radio receiver.

Uses of Radio Waves

The prime purpose of radio is to convey information from one place to another through the intervening media (i.e., air, space, nonconducting materials) without wires. Besides being used for transmitting sound and televisiontelevision,
transmission and reception of still or moving images by means of electrical signals, originally primarily by means of electromagnetic radiation using the techniques of radio, now also by fiber-optic and coaxial cables and other means.
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 signals, radio is used for the transmission of data in coded form. In the form of radarradar,
system or technique for detecting the position, movement, and nature of a remote object by means of radio waves reflected from its surface. Although most radar units use microwave frequencies, the principle of radar is not confined to any particular frequency range.
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 it is used also for sending out signals and picking up their reflections from objects in their path. Long-range radio signals enable astronauts to communicate with the earth from the moon and carry information from space probes as they travel to distant planets (see space explorationspace exploration,
the investigation of physical conditions in space and on stars, planets, and other celestial bodies through the use of artificial satellites (spacecraft that orbit the earth), space probes (spacecraft that pass through the solar system and that may or may not
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). For navigation of ships and aircraft the radio rangeradio range,
geographically fixed radio transmitter that radiates coded signals in all directions to enable aircraft and ships to determine their bearings. An aircraft or ship can determine its line of position and drift if it knows its bearing relative to the radio transmitter
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, radio compass (or direction finder), and radio time signals are widely used. Radio signals sent from global positioning satellites can also be used by special receivers for a precise indication of position (see navigation satellitenavigation satellite,
artificial satellite designed expressly to aid the navigation of sea and air traffic. Early navigation satellites, from the Transit series launched in 1960 to the U.S. navy's Navigation Satellite System, relied on the Doppler shift.
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). Digital radiodigital radio,
audio broadcasting in which an analog audio signal is converted into a digital signal before being transmitted; also known as digital audio broadcasting (DAB) and high-definition radio.
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, both satellite and terrestrial, provides improved audio clarity and volume. Various remote-control devices, including rocket and artificial satellite operations systems and automatic valves in pipelines, are activated by radio signals. The development of the transistortransistor,
three-terminal, solid-state electronic device used for amplification and switching. It is the solid-state analog to the triode electron tube; the transistor has replaced the electron tube for virtually all common applications.
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 and other microelectronic devices (see microelectronicsmicroelectronics,
branch of electronic technology devoted to the design and development of extremely small electronic devices that consume very little electric power. Although the term is sometimes used to describe discrete electronic components assembled in an extremely small
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) led to the development of portable transmitters and receivers. Cellular and cordless telephones are actually radio transceivers. Many telephone calls routinely are relayed by radio rather than by wires; some are sent via radio to relay satellites. Some celestial bodies and interstellar gases emit relatively strong radio waves that are observed with radio telescopes composed of very sensitive receivers and large directional antennas (see radio astronomyradio astronomy,
study of celestial bodies by means of the electromagnetic radio frequency waves they emit and absorb naturally. Radio Telescopes

Radio waves emanating from celestial bodies are received by specially constructed antennas, called radio telescopes,
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Transmission and Reception of Radio Waves

For the propagation and interception of radio waves, a transmitter and receiver are employed. A radio wave acts as a carrier of information-bearing signals; the information may be encoded directly on the wave by periodically interrupting its transmission (as in dot-and-dash telegraphy) or impressed on it by a process called modulationmodulation,
in communications, process in which some characteristic of a wave (the carrier wave) is made to vary in accordance with an information-bearing signal wave (the modulating wave); demodulation is the process by which the original signal is recovered from the wave
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. The actual information in a modulated signal is contained in its sidebandssideband,
any frequency component of a modulated carrier wave other than the frequency of the carrier wave itself, i.e., any frequency added to the carrier as a result of modulation; sidebands carry the actual information while the carrier contributes none at all.
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, or frequencies added to the carrier wave, rather than in the carrier wave itself. The two most common types of modulation used in radio are amplitude modulation (AM) and frequency modulation (FM). Frequency modulation minimizes noisenoise,
any signal that does not convey useful information. Electrical noise consists of electrical currents or voltages that interfere with the operation of electronic systems.
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 and provides greater fidelity than amplitude modulation, which is the older method of broadcastingbroadcasting,
transmission, usually using radio frequencies, of sound or images to a large number of radio or television receivers. In the United States the first regularly scheduled radio broadcasts began in 1920 at 8XK (later KDKA) in Pittsburgh.
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. Both AM and FM are analog transmission systems, that is, they process sounds into continuously varying patterns of electrical signals which resemble sound waves. Digital radio uses a transmission system in which the signals propagate as discrete voltage pulses, that is, as patterns of numbers; before transmission, an analog audio signal is converted into a digital signal, which may be transmitted in the AM or FM frequency range. A digital radio broadcast offers compact-disc-quality reception and reproduction on the FM band and FM-quality reception and reproduction on the AM band.

In its most common form, radio is used for the transmission of sounds (voice and music) and pictures (television). The sounds and images are converted into electrical signals by a microphone (sounds) or video camera (images), amplified, and used to modulate a carrier wave that has been generated by an oscillatoroscillator, electronic
, electronic circuit that produces an output signal of a specific frequency. An oscillator generally consists of an amplifier having part of its output returned to the input by means of a feedback loop; the necessary and sufficient condition for
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 circuit in a transmitter. The modulated carrier is also amplified, then applied to an antennaantenna
, in electronics, system of wires or other conductors used to transmit or receive radio or other electromagnetic waves (see radio); sometimes called an aerial. The idea of using an antenna was developed by Guglielmo Marconi (c.1897).
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 that converts the electrical signals to electromagnetic waves for radiation into space. Such waves radiate at the speed of light and are transmitted not only by line of sight but also by deflection from the ionosphereionosphere
, series of concentric ionized layers forming part of the upper atmosphere of the earth from around 30 to 50 mi (50 to 80 km) to 250 to 370 mi (400 to 600 km) where it merges with the magnetosphere, the region of the Van Allen radiation belts.
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Receiving antennas intercept part of this radiation, change it back to the form of electrical signals, and feed it to a receiver. The most efficient and most common circuit for radio-frequency selection and amplification used in radio receivers is the superheterodyne. In that system, incoming signals are mixed with a signal from a local oscillator to produce intermediate frequencies (IF) that are equal to the arithmetical sum and difference of the incoming and local frequencies. One of those frequencies is applied to an amplifier. Because the IF amplifier operates at a single frequency, namely the intermediate frequency, it can be built for optimum selectivity and gain. The tuning control on a radio receiver adjusts the local oscillator frequency. If the incoming signals are above the threshold of sensitivity of the receiver and if the receiver is tuned to the frequency of the signal, it will amplify the signal and feed it to circuits that demodulate it, i.e., separate the signal wave itself from the carrier wave.

There are certain differences between AM and FM receivers. In an AM transmission the carrier wave is constant in frequency and varies in amplitude (strength) according to the sounds present at the microphone; in FM the carrier is constant in amplitude and varies in frequency. Because the noise that affects radio signals is partly, but not completely, manifested in amplitude variations, wideband FM receivers are inherently less sensitive to noise. In an FM receiver, the limiter and discriminator stages are circuits that respond solely to changes in frequency. The other stages of the FM receiver are similar to those of the AM receiver but require more care in design and assembly to make full use of FM's advantages. FM is also used in television sound systems. In both radio and television receivers, once the basic signals have been separated from the carrier wave they are fed to a loudspeaker or a display device (now typically a liquid crystal display), where they are converted into sound and visual images, respectively.

Development of Radio Technology

Radio is based on the studies of James Clerk Maxwell, who developed the mathematical theory of electromagnetic waves, and Heinrich Hertz, who devised an apparatus for generating and detecting them. Guglielmo Marconi, recognizing the possibility of using these waves for a wireless communication system, gave a demonstration (1895) of the wireless telegraph, using Hertz's spark coil as a transmitter and Edouard Branly's coherer (a radio detector in which the conductance between two conductors is improved by the passage of a high-frequency current) as the first radio receiver. The effective operating distance of this system increased as the equipment was improved, and in 1901, Marconi succeeded in sending the letter S across the Atlantic Ocean using Morse codeMorse code
[for S. F. B. Morse], the arbitrary set of signals used on the telegraph (see code). It may also be used with a flash lamp for visible signaling. The international (or continental) Morse code is a simplified form generally used in radio telegraphy.
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. In 1904, Sir John A. Fleming developed the first vacuum electron tubeelectron tube,
device consisting of a sealed enclosure in which electrons flow between electrodes separated either by a vacuum (in a vacuum tube) or by an ionized gas at low pressure (in a gas tube).
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, which was able to detect radio waves electronically. Two years later, Lee de Forest invented the audion, a type of triode, or three-element tube, which not only detected radio waves but also amplified them.

Radio telephony—the transmission of music and speech—also began in 1906 with the work of Reginald Fessiden and Ernst F. W. Alexanderson, but it was not until Edwin H. Armstrong patented (1913) the circuit for the regenerative receiver that long-range radio reception became practicable. The major developments in radio initially were for ship-to-shore communications. Following the establishment (1920) of station KDKA at Pittsburgh, Pa., the first commercial broadcasting station in the United States, technical improvements in the industry increased, as did radio's popularity. In 1926 the first broadcasting network was formed, ushering in the golden age of radio. Generally credited with creating the first modern broadband FM system, Armstrong built and operated the first FM radio station, KE2XCC, in 1938 at Alpine, N.J. The least expensive form of entertainment during the Great Depression, the radio receiver became a standard household fixture, particularly in the United States. Subsequent research gave rise to countless technical improvements and to such applications as radio facsimilefacsimile
or fax,
in communications, system for transmitting pictures or other graphic matter by wire or radio. Facsimile is used to transmit such materials as documents, telegrams, drawings, pictures taken from satellites, and even entire newspapers.
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, radar, and television. The latter changed radio programming drastically, and the 1940s and 50s witnessed the migration of the most popular comedy and drama shows from radio to television. Radio programming became mostly music and news and, to a lesser extent, talk shows. The turn of the century saw a potential rebirth for radio as mobile digital radio entered the market with a satellite-based subscription service in Europe (1998) and in the United States (2000). Two years later, a land-based digital radio subscription service was inaugurated in the United States.

Radios that combine transmitters and receivers are now widely used for communications. Police and military forces and various businesses commonly use such radios to maintain contact with dispersed individuals or groups. Citizens band (CB) radios, two-way radios operating at frequencies near 27 megahertz, most typically used in vehicles for communication while traveling, became popular in the 1970s. Cellular telephonescellular telephone
or cellular radio,
telecommunications system in which a portable or mobile radio transmitter and receiver, or "cellphone," is linked via microwave radio frequencies to base transmitter and receiver stations that connect the user to a conventional
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, despite the name, are another popular form of radio used for communication.


See A. and W. Marcus, Elements of Radio (6th ed. 1973); D. L. Schilling, Principles of Communications Systems (2d ed. 1986).



a method of transmitting messages without wires over a distance by means of radio waves, invented by A. S. Popov in 1895. The term “radio” also refers to the scientific and technical field associated with the study of the physical phenomena that underlie this method (radio physics) and with the method’s use for communication (radio communication), sound broadcasting (radio broadcasting), the transmission of images (television), signaling, monitoring, and control (radio remote control), and the detection and location of various objects (radar).

In a limited sense, the term also signifies radio broadcasting, one of the greatest mass mediums for distributing political, cultural, educational, and general-interest information.

The term “radio” came into use in the second decade of the 20th century.



a monthly scientific journal of radio engineering for the general public, published by the Ministry of Communications and the DOSAAF (All-Union Voluntary Society for Cooperation With the Army, Air Force, and Navy) of the USSR.

Radio has been published since 1924 in Moscow, and the present title has been used since 1946 (up to 1931 the title was Radioliubitel’ [Radio Amateur], and from 1931 to 1941, Radiofront). The journal provides information on the most important advances in radio engineering, electronics, and communication; it serves the amateur-radio field and popularizes competitions for radio amateurs. Descriptions of commercial and amateur electronic instruments and equipment are contained in the journal, as are reference materials and the latest information on measuring techniques and household appliances. There is also advice on problems in radio engineering.

Radio has been awarded the Order of the Red Banner of Labor (1974). Circulation, 850,000 (1975).


The transmission of signals through space by means of electromagnetic waves.


Communication between two or more points, employing electromagnetic waves as the transmission medium.

Radio waves transmitted continuously, with each cycle an exact duplicate of all others, indicate only that a carrier is present. The message must cause changes in the carrier which can be detected at a distant receiver. The method used for the transmission of the information is determined by the nature of the information which is to be transmitted as well as by the purpose of the communication system.

In code telegraphy the carrier is keyed on and off to form dots and dashes. The technique, often used in ship-to-shore and amateur communications, has been largely superseded in many other point-to-point services by more efficient methods.

In frequency-shift transmission the carrier frequency is shifted a fixed amount to correspond with telegraphic dots and dashes or with combinations of pulse signals identified with the characters on a typewriter. This technique is widely used in handling the large volume of public message traffic on long circuits, principally by the use of teletypewriters.

In amplitude modulation the amplitude of the earner is made to fluctuate, to conform to the fluctuations of a sound wave. This technique is used in AM broadcasting, television picture transmission, and many other services.

In frequency modulation the frequency of the carrier is made to fluctuate around an average axis, to correspond to the fluctuations of the modulating wave. This technique is used in FM broadcasting, television sound transmission, and microwave relaying.

In pulse transmission the carrier is transmitted in short pulses, which change in repetition rate, width, or amplitude, or in complex groups of pulses which vary from group to succeeding group in accordance with the message information. These forms of pulse transmission are identified as pulse-code, pulse-time, pulse-position, pulse-amplitude, pulse-width, or pulse-frequency modulation. Such techniques are complex and are employed principally in microwave relay systems.

In radar the carrier is normally transmitted as short pulses in a narrow beam, similar to that of a searchlight When a wave pulse strikes an object, such as an aircraft, energy is reflected back to the station, which measures the round-trip time and converts it to distance. A radar can display varying reflections in a maplike presentation on a cathode-ray tube. See Radar

Hundreds of thousands of radio transmitters exist, each requiring a carrier at some radio frequency. To prevent interference, different carrier frequencies are used for stations whose service areas overlap and receivers are built to select only the carrier signal of the desired station. Resonant electric circuits in the receiver are adjusted, or tuned, to accept one frequency and reject others.

All nations have a sovereign right to use freely any or all parts of the radio spectrum. But a growing list of international agreements and treaties divides the spectrum and specifies sharing among nations for their mutual benefit and protection. Each nation designates its own regulatory agency. In the United States all nongovernmental radio communications are regulated by the Federal Communications Commission (FCC).


1. the use of electromagnetic waves, lying in the radio-frequency range, for broadcasting, two-way communications, etc.
2. an electronic device designed to receive, demodulate, and amplify radio signals from sound broadcasting stations, etc.
3. a similar device permitting both transmission and reception of radio signals for two-way communications


(1) The transmission of wireless signals (electromagnetic waves) over the air or through a hollow tube called a "waveguide." Although "radio" is often thought of as only AM and FM or sometimes two-way radio, all transmission systems that propagate signals through the air are some form of "radio," including TV, satellite, portable phones, cellphones and wireless LANs. See spectrum.

Electromagnetic Spectrum
The radio portion of the entire spectrum of radiation is from 3 kHz to 300 GHz. This huge band of frequencies has been defined by the FCC in the U.S. and governmental bodies in other countries.

(2) An electronic circuit that transmits and receives wireless signals (electromagnetic waves). The phrase "the device has four radios" means the unit has some combination of receivers and transceivers, which may reside on one or more chips. See transceiver.

A Four-Radio Chip From TI
In addition to a cellular radio, modern smartphones have other radios. This single chip has four: two receivers (FM and GPS) and two transceivers (Wi-Fi and Bluetooth). (Image courtesy of Texas Instruments, Inc.)

(3) A device that receives wireless audio signals (electromagnetic waves) such as an AM/FM radio or satellite radio. See Vintage Radio Museum.

For the Early Adopter!
In 1925, listeners tuned three stages on this battery-operated Stewart Warner Model 325 to get a good signal. Once they got one, they logged the numbers so they could tune in quickly the next time. (Equipment courtesy of Vintage Radio and Communications Museum of Connecticut,

A Double Thrill for Dad
Not only was it a pleasure for John Coolidge to have his son Calvin accept the Republican nomination for U.S. president in Cleveland, Ohio in 1924, but to actually hear the speech was quite exciting. (Image courtesy of Vintage Radio and Communications Museum of Connecticut,
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