radio

Radio transmission and reception. The radio frequency oscillator generates rapidly varying electrical signals, which are sent to the transmitting aerial. In the aerial, the signals produce radio waves (the carrier wave), which spread out at the speed of light. The sound signal is added to the carrier wave by the modulator. When the radio waves fall on the receiving aerial, they induce an electrical current in the aerial. The electrical current is sent to the tuning circuit, which picks out the signal from the particular transmitting station desired. The demodulator separates the sound signal from the carrier wave and sends it, after amplification, to the loudspeaker.

A photograph of the Italian wireless pioneer Guglielmo Marconi beside the telegraph on board his yacht Elettra in the early 1930s. While sailing, Marconi experimented with sending and receiving radio messages.

The Italian pioneer of radio telegraphy Guglielmo Marconi, speaking from his 700-ton yacht, Elettra in Genoa, Italy, to an audience in Sydney, Australia. The yacht, purchased in 1919, was converted into a floating laboratory where he tested short-wave reception and transmission. By the end of the 1920s he had set up a worldwide system of short-wave stations.

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.

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.