When an alternating current flows (backwards and forwards) in a wire, electromagnetic waves are produced. The wire acts as an aerial. When the waves reach another wire, they produce an alternating current there. This process makes communication using electromagnetic waves possible.
A simple AM (amplitude modulation) radio transmitter consists of a microphone, RF (radio frequency) oscillator, producing an alternating current of a fixed frequency, modulator and amplifier. When you speak into the microphone, you voice is changed into an alternating current. The current has the same frequency as your voice. The audio frequency (AF) of your voice is of the order of 1 kHz and variable but the frequency of the radio waves produced by the RF oscillator is about 1 MHz. The current produced by the voice is superimposed onto (modulated by) the signal produced by the RF. The resulting signal is then amplified and transmitted via the aerial.
In the receiving aerial, incoming electromagnetic waves generate electrical impulses. The impulses produced are due to all the electromagnetic waves received by the aerial. To pick out the waves from a particular source or of a particular frequency, the receiver must be tuned. The tuning circuit selects a particular frequency – typically the frequency a certain radio station transmits on. The demodulator removes the RF modulator, leaving only the AF signals. The AF signals are then amplified and turned into sound by the loudspeaker.
