satellite radio: see digital radio.

radio telescope: see radio astronomy.

radio 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 and the geographic location of the transmitter. By taking successive bearings on two or more radio ranges the craft can determine its geographic position. Radio ranges are usually unattended; they emit either repeated call letters or steady signals that are periodically interrupted by station identification letters in Morse code. The aircraft or ship obtains its bearings relative to the radio range by picking up these signals with a receiver having a directional antenna, usually a loop antenna. The strength of the signal received depends on the orientation of the antenna relative to the radio range. By varying the orientation of the antenna and observing the changes in signal strength, the bearing of the vehicle can be obtained. When the antenna is driven automatically, the instrument is called an automatic direction finder (ADF). Both manual and automatic direction finders are also called radio compasses, although in aircraft the radio compass usually means an ADF. Another type of radio range called an A-N range transmits two coded signals via directional antennas so that a pilot on one of four fixed courses hears a continuous tone in his or her receiver when the craft's bearing is correct; if it veers off course either a Morse A or N is heard depending on the direction in which the error is made. A very-high-frequency (VHF) omnidirectional radio range transmits a reference signal and another signal that varies from the reference according to the bearing of the receiver. Radio ranging is being made obsolete by the Global Positioning System (GPS), which uses a network of orbiting satellites to precisely locate the position of an aircraft or ship.

radio frequency, range of electromagnetic waves with a frequency or wavelength suitable for utilization in radio communication. Some of these waves serve as carriers of the lower-frequency audio waves; others are modulated by video or digital information. Short waves have relatively high frequencies; long waves have relatively low frequencies. Radio waves are identified by their frequencies, expressed in kilohertz (kHz), i.e., thousands of cycles per second, in megahertz (MHz), i.e., millions of cycles per second, or in gigahertz, i.e., billions of cycles per seconds. Signals in the amplitude modulation (AM) broadcast band have frequencies ranging from 540 to 1,800 kHz. Frequency modulation (FM) broadcast frequencies range from 88 MHz to 108 MHz. A range, or band, of radio frequencies is regularly assigned to a broadcasting station or service by the nation in which it operates. In the United States, the Federal Communications Commission is responsible for that task. Countries cooperate on on a worldwide basis through the International Telecommunication Union, which holds periodic conferences. See also the table entitled Radio Frequencies for the classification of radio frequencies.

radio compass: see radio range.

radio beacon: see radio range.

radio 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, whose use corresponds to that of the optical telescope in observing visible light. In the most common design, a parabolic "dish" replaces the mirror of the reflecting optical telescope. This dish serves to focus the radio waves into a concentrated signal that is then filtered, amplified, and finally analyzed using a computer. The radio signals received from outer space are extremely weak, and long observing times are required to collect a useful amount of energy. Therefore, most radio telescopes are mounted so that they can automatically track a given object as its position changes because of the rotation of the earth.

Galactic Sources of Radio Waves

Naturally occurring radio emission from the sky was accidentally discovered in 1931 by Karl Jansky. An inexplicable source of radio noise was identified in 1940 by Gröte Reber, using a radio telescope in the backyard of his home, as originating from our own galaxy, the Milky Way. This radiation is spread over a wide band of radio frequencies and originates in the ionized interstellar gases surrounding hot, bright stars. In these so-called H II regions, free electrons emit radio waves when they are scattered by collisions with the heavier ions. Other sources of radio waves within our galaxy are the remnants of supernovas, or exploding stars. The most famous example of a supernova remnant is the Crab Nebula in Taurus.

Because there are strong magnetic fields (see magnetism) in the vicinities of supernovas remnants, an additional mechanism is present for producing radio waves. This is the synchrotron radiation emitted by energetic electrons as they rapidly spiral around the magnetic lines of force, instead of simply being deflected by collisions with ions.

A third source of radio waves within our own galaxy consists of the atoms and molecules in the interstellar matter. This radiation is at discrete frequencies instead of over a broad band, or continuum, of frequencies. The first of these "radio lines" to be discovered was the line at a wavelength of 21 cm produced by the hydrogen atom (as opposed to the hydrogen molecule, which is composed of two atoms). The intensity of this line in the radiation from a given region is a direct measure of the amount of hydrogen there. Because hydrogen is a major constituent of the interstellar medium, the 21-cm line has provided astronomers with a means of mapping the spiral structure of the Milky Way. The visible light is blocked off by the same interstellar material in which the hydrogen giving rise to a 21-cm line lies, so that the view of the galaxy is obscured in certain directions, particularly in the direction of the center of the galaxy. Thus, before the advent of radio astronomy, the spiral structure of the Milky Way had not actually been observed but was only inferred from comparison with the Andromeda Galaxy and from other indirect studies. Besides atomic hydrogen, certain simple organic (carbon-based) molecules, including cyanogen (CN) and formaldehyde (H₂CO), have been discovered in the interstellar medium by means of their radio lines.

Extragalactic Sources of Radio Waves

Radio waves also come from outside the Milky Way. These extragalactic radio sources have great implications for cosmology, the theory of the overall structure of the universe. Spiral galaxies like the Milky Way are only weak sources of radio waves, but certain giant elliptical and irregular galaxies emit more than a million times as much radio energy as ordinary galaxies. Such galaxies are usually marked by dust lanes, which are unusual for galaxies lacking spiral arms. Some of these objects can be detected only by their radio emission, but in other cases the position of the radio source has been determined accurately enough to allow astronomers to identify the radio source with a galaxy visible in an image taken with a large optical telescope.

Other radio sources were optically identified with what at first appeared to be faint blue stars. However, it was discovered that these "stars" had enormous red shifts (shifting of the spectral lines toward the red end of the spectrum) that implied, according to Hubble's law, that they were the most remote objects ever detected and that their intrinsic intensities were about 1000 times greater than an entire galaxy. These extraordinary objects were named quasi-stellar radio sources, which was soon shortened to quasars. Their nature is still not completely understood.

Many thousands of extragalactic radio sources are known. Of those optically identified radio sources, roughly one third are quasars, and the remainder are radio galaxies. In addition to these localized radio sources, there is uniform low-level radio noise from every direction in the sky. This cosmic background radiation is believed to be an indication that the universe began with an explosive big bang rather than having always existed in an unchanging steady state. More recently radio astronomy has discovered pulsars, thought to be rapidly spinning neutron stars that radiate bursts of energy on and off regularly between 1 and 30 times a second.

radio altimeter: see altimeter.

radio, transmission or reception of electromagnetic radiation in the radio frequency 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 television signals, radio is used for the transmission of data in coded form. In the form of radar 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 exploration). For navigation of ships and aircraft the radio range, 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 satellite). Digital radio, 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 transistor and other microelectronic devices (see microelectronics) 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 astronomy).

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 modulation. The actual information in a modulated signal is contained in its sidebands, 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 noise and provides greater fidelity than amplitude modulation, which is the older method of broadcasting. 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 oscillator circuit in a transmitter. The modulated carrier is also amplified, then applied to an antenna 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 ionosphere.

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 (usually a cathode-ray tube), 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 code. In 1904, Sir John A. Fleming developed the first vacuum electron tube, 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 facsimile, 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 telephones, despite the name, are another popular form of radio used for communication.

digital 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. Digital radio reception is virtually free of static and fading, pops, and hisses; overall, adjacent stations do not interfere within one another, audio clarity and volume are improved, and weather, noise, and other interference cease to be a factor. Digital radio can be land based (or terrestrial) or transmitted via satellite. In either case, a special receiver is required to decode the multiplexed signal; the receiver may contain a small display that provides information about the audio content (such as the name of the artist or title of the music).

The land-based technology was first deployed in Great Britain in 1995, and has since become established throughout Europe. The first satellite-based digital radio system was WorldSpace, which orbited the first of its three geostationary earth orbit (GEO) satellites, AfriStar, in 1998. Each satellite transmits three overlapping signal beams carrying more than 40 channels of programming; most of world (except mainly North America and Australia) is covered. The first satellite-based system to provide a mobile subscription digital radio service covering the United States was XM Satellite Radio, which orbited two GEO satellites in 2001. XM's ground station transmits digital signals to its satellites, which retransmit them directly to radio receivers on the ground. The receivers unscramble the signal, which contain up to 100 channels of digital audio. In metropolitan areas where tall buildings, overpasses, and other obstacles can interfere with the signals when, for example, the receiver is in a moving vehicle, a network of ground-based repeaters retransmit the signals. The receiver also buffers the signal briefly so that if it loses the satellite signal it can use one from a repeater to maintain a continuous broadcast. Sirius Satellite Radio, which launched national service to the United States in 2002, employs three satellites in inclined elliptical orbits instead of GEO satellites.

citizens band radio: see radio.

Frequency (kHz)	Name	Abbr.
10-30	Very low	VLF
30-300	Low	LF
300-3,000	Medium	MF
3,000-30,000	High	HF
30,000-300,000	Very high	VHF
300,000-3,000,000	Ultra high	UHF
3,000,000-30,000,000	Super high	SHF
30,000,000-300,000,000	Extremely high	EHF

Radio Free Europe (RFE), broadcasting organization established in 1950 with the stated mission of promoting democratic values and institutions. Its original purpose was to broadcast news to countries behind the "Iron Curtain" during the cold war. In 1975, it was merged with Radio Liberty (RL), a similar enterprise that broadcast to the nations inside the Soviet Union. RFE receives most of its funding from the U.S. Congress. Until 1971, the funds were channeled through the Central Intelligence Agency; since that time the funds have been received in the form of grants through the Broadcasting Board of Governors of the U.S. Information Agency. The collapse of the USSR brought about changes including budget cuts and the relocation of headquarters from Munich, Germany, to Prague, the Czech Republic, in 1995. Broadcasts were ended in some areas but added in others. They are now sent to E and SE Europe, Russia, the Caucasus, Central Asia, and the Middle East. They continue to include news, political commentaries, sports, and music, and to be written, produced, and broadcast by nationals from the audience countries. RFE/RL now broadcasts over shortwave, AM/FM channels, and the Internet.

Radio City: see Rockefeller Center.

Nuffield Radio Astronomy Laboratories: see Jodrell Bank Observatory.

National Radio Astronomy Observatory (NRAO), federal observatory for radio astronomy, founded in 1956 and operated under contract with the National Science Foundation by Associated Universities, Inc., a group of major universities. The headquarters are at Charlottesville, Va.; the original observatory site is in Greenbank, W.Va., where the antennas, or radio telescopes, include a fully steerable 140-ft (43-m) paraboloid; an interferometer consisting of three steerable 85-ft (26-m) paraboloids; a horn-shaped antenna 120 ft (37 m) in length that is fixed in place; and two smaller, steerable paraboloids; a modern 328-ft (100-m) fully steerable telescope is under construction. At Kitt Peak, near Tucson, Ariz., NRAO has a 36-ft (11-m) steerable paraboloid; near Socorro, New Mexico, the NRAO's Very Large Array (VLA) consists of 27 parabolic dishes, each 82 ft (25 m) in diameter, mounted on a Y-shaped track with arms up to 14 mi (21 km) long. Finally, the observatory operates the Very Baseline Array (VLBA) consisting of ten radio telescopes placed around the earth that operate in unison. Principal research programs of the NRAO include the study of galactic structure, extragalactic radio sources, molecules in space, pulsars, quasars, and the evolution of stars and galaxies. Astronomers using the VLA have discovered filaments, jets, and high-temperature features in the center of our own galaxy and in extragalactic radio sources that may help explain the high energy of quasars. The system allows the study of the nuclei of active galaxies and helps determine distances to radio sources more accurately.

Museum of Television and Radio: see Paley Center for Media.

CB radio: see radio.

Combination of radio receiver and antenna, used for observation in radio and radar astronomy. Radio telescopes vary widely, but all have two basic components: a large radio antenna or an antenna array and a radiometer or radio receiver. Because some astronomical radio sources are extremely weak, radio telescopes are usually very large, and only the most sensitive radio receivers are used. The first large fully steerable radio telescope was completed in 1957 at Jodrell Bank, Eng. The world's largest fully steerable radio telescope is the 360 × 330-ft (110 × 100-m) off-axis antenna operated by the National Radio Astronomy Observatory in Green Bank, W.Va. The largest single radio telescope is the 1,000-ft (305-m) fixed spherical reflector at the Arecibo Observatory in Puerto Rico. The world's most powerful radio telescope is the Very Large Array in New Mexico, made up of 27 separate mobile parabolic antennas that together provide the angular resolution of a single antenna 22 mi (35 km) in diameter.

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Broadcast serial drama, characterized by a permanent cast of actors, a continuing story, tangled interpersonal situations, and a melodramatic or sentimental style. Its name derived from the soap and detergent manufacturers who originally often sponsored such programs on radio. Soap operas began in the early 1930s as 15-minute radio episodes and continued on television from the early 1950s as 30-minute and later hour-long episodes. Usually broadcast during the day and aimed at housewives, they initially focused on middle-class family life, but by the 1970s their content had expanded to include a wider variety of characters and situations and a greater degree of sexual explicitness. In the 1980s similar series began to be aired in prime-time evening hours (e.g., Dallas and Dynasty). Seealso Carlton E. Morse; Irna Phillips.

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Study of celestial bodies by measuring the energy they emit or reflect at radio wavelengths. It began in 1931 with Karl Jansky's discovery of radio waves from an extraterrestrial source. After 1945, huge dish antennas, improved receivers and data-processing methods, and radio interferometers let astronomers study fainter sources and obtain greater detail. Radio waves penetrate much of the gas and dust in space, giving a much clearer picture of the centre and structure of the Milky Way Galaxy than optical observation can. This has allowed detailed studies of the interstellar medium in the Galaxy and the discovery of previously unknown cosmic objects (e.g., pulsars, quasars). In radar astronomy, radio signals are sent to near-Earth bodies or phenomena (e.g., meteor trails, the Moon, asteroids, nearby planets) and the reflections detected, providing precise measurement of the objects' distances and surface structure. Because radar waves can penetrate even dense clouds, they have provided astronomers' only maps of the surface of Venus. Radio and radar studies of the Moon revealed its sandlike surface before landings were made. Radio observations have also contributed greatly to knowledge about the Sun. Seealso radio telescope.

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Electromagnetic radiation of lower frequency (hence longer wavelength) than visible light or infrared radiation, and consisting of the range of frequencies used for navigation signals, AM and FM broadcasting, television transmissions, cell-phone communications, and various forms of radar. For radio transmission, information is imparted to a carrier wave by varying (modulating) its amplitude, frequency, or duration. The technology of radio arose from the work of Michael Faraday, James Clerk Maxwell, Heinrich Hertz, Guglielmo Marconi, and others, and improvement followed the development of the vacuum tube, the electronic-tube oscillator, the tuned circuit, and other components. Later innovations have included the replacement of tubes by transistors and of wires by printed circuits. Seealso radio and radar astronomy.

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