RADIO

transmission and detection of communication signals consisting of electromagnetic waves that travel through the air in a straight line or by reflection from the ionosphere or from a communications satellite. transmission and detection of communication signals consisting of electromagnetic waves that travel through the air in a straight line or by reflection from the ionosphere or from a communications satellite. Two characteristics of electromagnetic-wave motion are most significant: the physical length of the wave and the number of times the wave cycle is repeated in a given period of time. The unit used to measure cycles per second is called the hertz (Hz); frequencies used in radio transmissions range up to 30 gigahertz (30,000,000,000 Hz). The higher the frequency, the smaller the wavelength, since all the waves travel at the speed of light (about 186,000 mi [300,000 km] per second). For radio transmission, information is imparted to a carrier wave by varying its amplitude, frequency, or duration in a process called modulation. Amplitude modulation (AM) creates side-band frequencies at the upper and lower limits of a carrier wave, which register variations in the strength of the wave. Frequency modulation (FM) varies the number of cycles the wave goes through, instead of its amplitude. These changes in the otherwise constant frequency of the wave carry the desired information from transmitter to receiver. Duration modulation is a simpler system, in which a constant tone is transmitted for a certain period of time, shut off, and then transmitted again. Morse code utilizes this system. Radio first became a possibility when the English physicist Michael Faraday demonstrated that an electrical current could produce a magnetic field. In 1864 James Clerk Maxwell, a professor of experimental physics at Cambridge, proved mathematically that these electrical disturbances could be detected at considerable distances. Maxwell predicted that this electromagnetic energy could move outward in waves traveling at the speed of light. In 1888 Heinrich Hertz demonstrated that Maxwell's prediction was true for transmissions over short distances. The Italian physicist Guglielmo Marconi then perfected a radio system that in 1901 transmitted Morse code over the Atlantic Ocean. Next came the development of the vacuum tube, which amplified (strengthened) the radio signal that was received at an antenna; thus, much weaker signals could be transmitted and received than had previously been possible. It was next discovered that the electric current in a vacuum tube could be made to oscillate. An electronic-tube oscillator was thus able to generate very pure radio waves. Reception was also improved with the refinement of the tuning circuit. These and other components needed to produce radio receivers of acceptable quality underwent rapid improvement in the period before World War II. Individual broadcasting stations were assigned a portion of an arbitrary frequency scale, so that the signal of one station would not conflict with another. Other frequency ranges have been reserved for the many additional uses of radio signals, which include navigational aids for ships and aircraft, two-way voice transmission, and space and satellite communications. Innovations after the war, especially the replacement of tubes by transistors and of wires by printed circuits, drastically reduced the amount of power the receiver needed to operate and allowed its components to be miniaturized. Other advances included improvements in the sound fidelity of transmitting and receiving equipment and the perfection of FM stereo broadcasting. Additional reading Robert L. Hilliard (ed.), Radio Broadcasting: An Introduction to the Sound Medium, 3rd ed. (1985), addresses such aspects as broadcasting, facilities, programming, production, and performance. J.G. Crowther, Discoveries and Inventions of the 20th Century, 5th ed. (1966), includes information on the early developments in radio transmission and reception. Reference Data for Engineers: Radio, Electronics, Computer, and Communication, 7th ed. (1985), contains a wealth of technical information. Kenneth Reginald Sturley, The Editors of the Encyclopdia Britannica