Radio Waves and Circuits

When an alternating current drives a transmitting aerial, charges in the aerial accelerate back and forth. Their changing distribution produces changing electric and magnetic fields. These changing fields travel away from the aerial as an electromagnetic wave. If the electrical oscillation is at a radio frequency, the emitted electromagnetic wave is a radio wave.

The radio wave has the same frequency as the oscillation that produced it: if the charges complete more oscillations each second, the transmitted wave has a higher frequency. The wave transfers energy away from the transmitter; the charges themselves do not travel through the space from one aerial to another.

Radio waves producing electrical oscillations

The process can also operate in reverse. When a radio wave reaches a receiving aerial, its changing electric field exerts forces on electrons in the metal. The electrons are driven backwards and forwards, producing an alternating potential difference and an alternating current in the connected receiving circuit. The radio wave has therefore induced electrical oscillations.

The induced oscillation has the frequency of the incoming radio wave. A receiver can be tuned so that it responds strongly to the desired frequency while responding much less to others. Other components can then amplify and process the weak electrical signal, for example to reproduce sound in a radio.

One reversible connection

The central relationship is reciprocal:

At the transmitter, electrical energy is converted into energy carried by radio waves. At the receiver, some of the radio-wave energy is transferred to electrons in the aerial and becomes an electrical signal. This is the physical link that makes radio broadcasting and other radio communication possible.