4.3. PROPAGATION OF RADIOWAVES

You know that the function of an aerial is to radiate electro-magnetic energy into space. Once this energy is released from the aerial, it will travel through space until it is picked up by the receiving aerial or until it stikes an object and is reflected off it, as it is the case with radar transmissions. It is therefore important for you to know what happens to a radiated wave in space

- what its path is,
- if it is absorbed by the earth,
- if it is reflected by the sky and so on.

fig. 21

In order that you will be able to tell how far the wave will travel before it can be picked up. The subject of what happens to a radiated electro-magnetic wave once it leaves the aerial is called the theory of WAVE PROPAGATION. When a radiated wave leaves the aerial, part of its energy travels along the earth, following the curvature of the earth. This is called the GROUND WAVE. Other waves which strike the ground between the transmitter and the horizon are called SPACE WAVES; and those which leave the aerial at an angle bigger than that between the aerial and the horizon are called SKY WAVES. The ground wave, the space waves and the sky waves all cary the transmitted intelligence.

But at certain frequencies one of the wave-types will be much more effective in transmitting the intelligence than will the others.

At comparatively low frequencies, most of the radiated energy is in the ground wave. Since the earth is a poor conductor, the ground wave is rapidly reduced, or “attenuated”, by absortion and is therefore not effective for transmissions over great distances unless large amounts of transmitted power are used.

The medium and long wave-band broadcast frequencies are examples of transmissions using ground waves. At these frequencies the effective radiating area usually lies within 200 miles radius from the transmitter. Stations more than 400 miles away from each other can therefore transmit on the same low frequencies, and yet not interfere with each other.

SKY WAVES AND GROUND WAVES

At first sight, one would think, that sky waves can serve no useful purposes, since they will only travel straight out into space and get lost.

For very high frequencies, this actually happens, and the skywaves is useless. But below a certain critical frequency the skywave does not travel into space: it is bent back to earth in the upper layers of the atmosphere.

This returning wave is not sharply reflected, as is light from a mirror. It is bent back slowly, as if it were going round a curve: it is therefore called a refracted wave.

This refracted wave, once it returns to earth, is reflected back into the sky again where it is once again refracted back to earth. This process of refraction from the sky and reflection from the earth continues until the wave is completely attenuated - the energy of the radiated wave dropping as its distance from the transmitting areal increases. A receiving aerial will be able to pick up a signal at any point where the refracted wave hits the earth. If the sky wave were radiated to the sky at one angle only, of course, no signal would arrive at any points save. Sky waves, however are radiated from the transmitter at many angles, there are therefore large areas of the earth's surface at which reception of signals form a particular transmitter as possible.

As the angle of radiation of the sky wave gets steeper, a point is eventually reached at which the wave is not longer refracted back to earth, but continues travelling into space. As a result, there is a zone around the aerial in which no refracted sky wave hits the earth.

Since the ground wave itself is only effective over a short distance, there exists a zone between the maximum effective radiating distance of the ground wave and the point where the first sky wave is refracted back to earth, which is an aerea of RADIO SILENCE in which no signals from this particular transmitter are received. This zone is called the SKIP DISTANCE.

The critical frequency, which is the frequency above which no sky wave (whatever its angle of radiation) can return to earth, varies - depending on numerous factors such as the time of day, the time of year, the weather, and others.

THE SPACE WAVE

At frequencies above the critical frequency, neither the ground wave nor the sky wave can be used for transmission. At these high frequencies, the ground wave is rapidly attenuated, and the sky wave is not refracted back to earth.

The only radiated wave which can be used for transmission at these frequencies is one that travels in a direct line from the transmitting aerial to the receiving aerial.

fig. 23

This type of transmission is called LINE-OF-SIGHT TRANSMISSION; and the radiated wave is called a SPACE WAVE.

Line-of-sight transmission is used in RADAR for detecting enemy aircraft, and in ship-to-plane communication. The frequencies used are usually above 3b megacycles.

FADING

Sometimes a receiving aerial picks up two signals which have travelled along different paths but originated from the same transmitting aerial. One signal will travel direct from the aerial; the other may have been reflected to the receiver off, say, an aeroplane.

fig. 24

Since the relative length of the paths of these signals is constantly changing, the two signals will sometimes be in phase, and at other times out of phase - thus tending either to cancel or to reinforce one another. The result is a variation in signal strength at the receiver end which is called FADING.