radio

radio Broadcasting or reception of electromagnetic radiation in the form of radio waves. A transmitter generates a radio signal of fixed frequency (the carrier wave). A microphone converts the sound to be broadcast into a varying electrical signal that combines with the carrier by means of modulation (varying the frequency or amplitude of the carrier). Frequency modulation (FM) minimizes interference and provides greater fidelity than amplitude modulation (AM). The modulated carrier wave passes to an aerial, which transmits it into the atmosphere. At the receiver, another aerial intercepts the signal, and it undergoes ‘detection’, the reverse of modulation, to retrieve the signal. Radio waves travel at the speed of light and are transmitted not only by line-of-sight (ground waves), but also by reflection from the ionosphere (sky waves). Sky waves enable long-range transmission. The ultra high frequency (UHF) and very high frequency (VHF) radio waves used to send signals for television penetrate the ionosphere with little reflection, and long-range broadcasting is made possible by means of artificial satellites. The development of radio was a truly international effort. It stems from the work of Scottish physicist James Clerk Maxwell. German physicist Heinrich Hertz devised an apparatus for the transmission and detection of radio waves. In 1895, Italian physicist Guglielmo Marconi gave a demonstration of the first wireless telegraph, and in 1901 he sent the first transatlantic message using Morse code. In 1904, English engineer Sir John Fleming invented the thermionic valve. In 1906, US physicist Lee De Forest developed the audion triode valve, which was able to detect and amplify radio waves. It remained at the heart of radio and television manufacture until the invention (1948) of the transistor.