1. A communication system based on broadcasting electromagnetic waves; SYN. wireless.
2. Medium for communication; SYN. radiocommunication, wireless.
Transmission and reception of radio waves. In radio transmission a microphone converts sound waves (pressure variations in the air) into electromagnetic waves that are then picked up by a receiving aerial and fed to a loudspeaker, which converts them back into sound waves.
The theory of electromagnetic waves was first developed by Scottish physicist James Clerk Maxwell 1864, given practical confirmation in the laboratory 1888 by German physicist Heinrich Hertz, and put to practical use by Italian inventor Guglielmo Marconi, who in 1901 achieved reception of a signal in Newfoundland transmitted from Cornwall, England.
To carry the transmitted electrical signal, an oscillator produces a carrier wave of high frequency; different stations are allocated different transmitting carrier frequencies. A modulator superimposes the audiofrequency signal on the carrier. There are two main ways of doing this: amplitude modulation (am), used for long- and medium-wave broadcasts, in which the strength of the carrier is made to fluctuate in time with the audio signal; and frequency modulation (fm), as used for vhf broadcasts, in which the frequency of the carrier is made to fluctuate. The transmitting aerial emits the modulated electromagnetic waves, which travel outward from it.
In radio reception a receiving aerial picks up minute voltages in response to the waves sent out by a transmitter. A tuned circuit selects a particular frequency, usually by means of a variable capacitor connected across a coil of wire. A demodulator disentangles the audio signal from the carrier, which is now discarded, having served its purpose. An amplifier boosts the audio signal for feeding to the loudspeaker. In a superheterodyne receiver, the incoming signal is mixed with an internally-generated signal of fixed frequency so that the amplifier circuits can operate near their optimum frequency.
1. Electromagnetic waves longer than about 0.3 mm (frequencies lower than about 1 THz). Radio is used to transmit a wide variety of signals, using various frequency ranges and types of modulation, such as am and fm broadcasts, microwave relays, and television broadcasts. See also hertz, radio frequency.
2. Audio signals transmitted over the Internet of quality comparable to those broadcast by commercial radio stations. See also Internet Talk Radio, mbone, RealAudio.

(Combining form) Indicating radiation or radioactivity
1. Radiant energy; radiation
2. Radioactive
3. Radium; X-rays
4. Radioactive isotopes especially as produced artificially
5. Radio

To transmit messages via radio waves