radio

Transmission and reception of radio waves. In radio transmission a microphone converts sound waves (pressure variations in the air) into a varying electric current, which is amplified and used to modulate a carrier wave which is transmitted as electromagnetic waves, which are then picked up by a receiving aerial, amplified, 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 in 1864, given practical confirmation in the laboratory in 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, Canada, 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 audio-frequency 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 outwards from it.