Radio Propagation

The radio system uses propagation of electromagnetic waves through space, to send information. Being an electromagnetic wave, the radio frequency (r.f.) wave has the same properties as visible light. The r.f. wave is, however, of a much lower frequency and so longer wavelength. The shorter waves in the electromagnetic spectrum have the highest frequency and the longer waves the lowest.

Frequency, or number of cycles per second, is given the unit of the hertz (Hz), named after the German radio pioneer Heinrich Hertz. Radio waves range from a few kilohertz to several gigahertz.

Waves of visible light are much shorter, ranging from the smallest visible wavelength for violet, about 40 millionths of a centimetre (16 millionths of an inch), to 75 millionths of a centimetre (about 30 millionths of an inch) for red. In a vacuum, all electromagnetic waves travel at a uniform speed of about 300,000 km (about 186,000 miles per second).

Radio waves are used in many other applications such as television, telephone transmission, radar, navigational systems and space communications. The table below shows various radio wavelenths in the electromagnetic spectrum.

The Radio Wave

Electromagnetic waves travel in straight lines and because the earth's surface is approximately spherical, long-distance radio communication is made possible by the reflection of radio waves from the ionosphere (Sky waves in the diagram below). Radio waves shorter than about 10m (about 33 ft) in wavelength designated as very high, ultrahigh, and superhigh frequencies (VHF, UHF, and SHF)are usually not reflected by the ionosphere normally passing through it and becoming lost to space (Space waves in the diagram below); thus,signal transmission at such very short wavelengths are received only within line-of-sight distances.

Wavelengths shorter than a few centimeters are absorbed by water droplets or clouds; those shorter than 1.5 cm (0.6 in) may be absorbed selectively by the water vapor present in a clear atmosphere.

The Ground wave hugs the surface of the earth and attenuates quite quickly owing to absorption. The wave operates at v.l.f., l.f. and m.f. frequencies

The sky wave is deflected back to the earth by the ionospere, which allows the signal field to carry over much larger distances than the ground wave. At particular frequencies the sky wave can be reflected completely around the planet by reflecting between the ionospere and the ground.

Another type of wave is the Space wave, responsible for v.h.f. and u.h.f. reception. The wave can penetrate the ionospere and so is lost to space. Signal transmission at these frequencies is normally line of sight.