Radio signals are transmitted using a carrier wave. Radio waves form part of the electromagnetic spectrum. Radio waves with the longest wavelengths are bounced off a layer high up in the Earth’s atmosphere, called the ionosphere. In this way radio messages can be bounced for very long distances.

A radio transmitter changes, or modules a radio wave in order to convey information. In AM radio the height of the carrier wave is altered according to the sound picked up by a microphone. In FM radio the frequency, or distance between the peaks in the radio wave, is changed. The radio receiver picks these signals, amplifies and then decodes them. If the signal is weak, AM radio can sound crackly, which is why it is being replaced by FM radio, which gives much clearer reception.