High above the surface of the Earth, some 60 miles or more above our heads, lies a region known as the ionosphere. At these great heights, the atmosphere is very thin, and as the ultra-violet radiation from the sun reacts with the well-spaced molecules of air, ions are produced, tiny particles with a small electrical charge. Until the era of the artificial satellite arrived, this layer of charged particles played an important part in radio communications.
In December 1901 Guglielmo Marconi built a powerful wireless set, and succeeded in sending a signal from Cornwall in England to Newfoundland on the east coast of Canada. The scientific world was quite astounded. Radio waves were well known to travel in a straight line, and for this reason, in normal atmospheric conditions, radio reception is not possible much beyond the horizon.
As it happens, the waves are slightly bent - or refracted - by the atmosphere, so the so-called radio horizon is somewhat further away than the optical horizon.
Not by any stretch of the imagination, however, should Marconi's radio signal have carried more than 2,000 miles across the Atlantic, but it did.
It was a man called Oliver Heaviside who proposed the answer. He deduced the existence of a shell of ions around the Earth which acted like a gigantic mirror and reflected the radio waves back down, allowing them to be picked up a great distance from the original point of transmission.
Later investigations showed there were a number of such layers, each differing from the other in the type and number of the ions they contained. The most important two became known as the Heaviside and Appleton layers.
The extent to which radio waves pass through a particular layer of ions, or are reflected by it, depends upon the wavelength of the signal.
By choosing an appropriate wavelength, one which they knew would be efficiently reflected, operators in the years following Marconi's great experiment were able to achieve radio contact over very large distances, such as from Paris to New York, or even farther.
But the ions only reflect the relatively long wavelengths used in ordinary radio broadcasts: shorter waves, such as those used for television, pass through the barrier unaffected, so the Heaviside layer is of little use in improving their range.
Nowadays, of course, intercontinental radio and television communications are conducted via satellite. Very short wavelengths are used, which allows the waves to pass through the ionosphere undisturbed, to be redirected by the orbiting satellite to their ultimate destination.