Chapter 6 - How Traditional Radio Works

Abstract

Radio waves refer to the electromagnetic waves that are transmitted across space (not just through air), as distinct from waves conducted along wires or in tubes (waveguides). The frequency of a wave is the number of oscillations per second, and this causes a wave to have a wavelength, which is found from speed/frequency. A radio wave is most efficiently radiated from a metal antenna whose length is a suitable fraction of a wavelength, such as half wave, and reception of radio waves is the best when the receiving antenna is also a suitable fraction of a wavelength. Anything that can generate electrical oscillations can form the basis of a transmitter, and early designs used sparks, generating a huge mixture of wavelengths. Later designs used mechanical generators for low-frequency waves that required huge antennae. Modulating a radio carrier wave always causes sidebands to appear. For amplitude modulation, the difference between a sideband frequency and the carrier frequency is equal to the modulating frequency. Early radio receivers for AM signals consisted only of an antenna, a crystal, and headphones. This arrangement could be used close to a transmitter, and when a coil and capacitor were added, it became possible to tune to more than one transmitter signal. The solution to the problems of medium-wave radio was supersonic heterodyne (“superhet”) receiver. The principle is to eliminate as far as possible the amplification of the carrier frequency, so that variable tuning is used only at the start of the receiver block.