Radio

The history of radio began in 1887 when scientist Heinrich Hertz (1857-1894), from Hamburg Germany, discovered how to generate and detect radio waves. This led others to investigate the mysterious waves and in 1896 Italian Guglielmo Marconi (1874-1937), patented the first practical system of wireless telegraphy. In 1901 Marconi managed to send a signal across the Atlantic Ocean.

At first, the radio was used as only a means of transmitting Morse code signal. Then following the invention of the triode valve in 1906, it became possible to transmit actual sounds also. So messages could be spoken instead of having to translate into Morse code. Broadcast programs of speech and music followed, and radio became a means of mass communication.

The most important part of the radio is the oscillator. This generates a signal that can be radiated from an aerial as radio waves. To transmit Morse code, which is made up of tiny dots and dashes, the oscillator is switched on and off so that the radio waves are transmitted in bursts. A short burst corresponds with the dot in the code, and a long burst represents the dash. To transmit sound, the sound is first converted into an electrical sound by means of a microphone.. Then sound signal is then used to modulate, or vary, the signal produced by the oscillator. After modulation, the oscillator signal is radiated from the aerial as before, but in this case, the radio waves are continuous. The oscillator signal is then made to carry the sound signal. For this reason, the signal from the oscillator is called a carrier, as the carry the sound signal.

A carrier can be modulated in several ways. The most common are amplitude modulation (AM) and frequency modulation (FM). An unmodulated carrier consists of waves generated at a frequency, or rate, of thousands and millions each second. The amplitude or strength of the carrier is constant.

In other words each wave is in the same height as the next. In amplitude modulation the amplitude of the carrier is made to vary according to the strength of the sound signal. At the receiver these variations are detected and reconverted into a sound signal. This is then reproduced through a loud speaker. Reception of the amplitude-modulated signal is sometimes spoiled by electrical interference. This becomes superimposed on the transmitted carrier, causing unwanted amplitude modulation. The receiver cannot distinguish between wanted and unwanted amplitude modulation, so it converts the interference into crackles and hisses. In frequency modulation, the sound signal varies the carrier frequency. In addition, the receiver only detects frequency changes. Therefore, the amplitude changes caused by interference are not detected and have no noticeable effect.