HOW SIGNALS FADE

Sun (Auckland), Volume IV, Issue 1031, 23 July 1930, Page 7

HOW SIGNALS FADE

EFFECT ON BROADCASTS Signal fading and the effects which are associated with it are unpleasantly familiar with the majority of wireless listeners, but few listeners really understand how fading is caused or, indeed, the conditions which affect the propagation of signals from broadcast stations. To understand how wireless wares travel, and from this to develop an understanding of how fading occurs, it is necessary to review briefly what is happening w'hen a transmitter is in operation. Broadly the station can be regarded as a point on the earth's surface, and the radiation can be imagined as a 'sort of hemisphere of electrical force, centred upon it and expanding out in every direction. Although this hemisphere expands in all directions from the station, since the waves travel in straight lines and cannot pass any distance through the ground, it is clear that normally they could not extend below the horizon about the station. Actually the earth is an electrical conductor, and there is a tendency for a certain amount of such a radiation to be bound to its surface —in effect, to flow along it. This effect is applied particularly in the “carrier wave’’ telephone system, where the waves are actually guided along the conducting wires. It will thus be seen that part of the radiation from the transmitter will move along over the ground in all directions from the transmitter.

ABSORPTION OF RADIATION The earth itself, but particularly objects on it, possesses the power of absorbing the energy of the radiation, so that when it has travelled a certain distance it has become so attenuated that it is of no value for signalling. The distance which this so-called “ground wave” will travel depends on tw r o factors. The first of these is the obvious one of the power of the transmitter.

The second limiting factor is the wave length of transmission. A long wave is absorbed by the earth’s surface much less rapidly than a short one, and in consequence there is a tendency for the ground wave of a long-w'ave station to extend farther than that from a short-wave station. It will thus be clear that if no other factor operated the range of any wireless transmitter would depend entirely on the distance its ground wave could be forced over the earth’s surface, and there would be a definite advantage in using a wave length as long as other considerations permitted. There is, however, a second most important influence which governs the behaviour of wireless signals. A large part of the radiation from every station travels upward from the aerial, and since the normal path of the waves is a straight line it might be supposed that they would simply escape into the free space above the earth. The sun, however, has an important effect on the upper part of the earth’s atmosphere. It creates a special electrical condition in the upper part of the air which varies according to the time of the day or night. In daylight the effect of this condition of the air may be disregarded. It has np appreciable effect on the upward moving waves from a broadcast station.

At night, however, the electric layer in the upper air creates a barrier through which the waves from a broadcast station cannot pass. On reaching the layer they penetrate it slightly, and in doing so they are bent in a downward direction, so that they ultimately pass out of the layer again and return to earth. An obvious effect, of this is that the transmission of signals to points below the horizon from a station is greatly facilitated, and hence the range of a station greatly increased. This effect actually does operate, and it explains the fact that distant stations may be heard readily at night, although they cannot be picked up in daylight. There is, however, another important effect of the downward bending of the waves. These waves come to earth at all points beyond about 70 miles from ordinary broadcast stations. TWO SETS OF WAVES The ground wave from the same stations extends to as much as 250 miles from these stations. Thus at night receiving sets more than 70 and less than 250 miles from a station pick up two distinct sets of waves — the ground wave and the sky wave. At first sight this might appear to be an advantage. Actually, however, it is the reverse. If the two sets of waves exactly coincided at any receiving point—that is, if the “trough” of the sky wave occurred at that point concurrent w T ith the “trough” of the ground wave, all would be well and a signal of double normal strength would be heard. Actually, this seldom occurs, and in many cases the trough of one wave occurs concurrently with the peak of the other. When this happens one wave virtually cancels the other, and the result is that nothing is heard in the receiver. Iu other words the signal fades. It is found that the nature of the electrified upper atmosphere is constantly changing. The result is that the path of sky waves also changes, and as a consequence they reach receiving posts on the ground alternately in step and out of step with the ground wave. There is thus a periodic rise and fall in signal strength at receivers within the fading area.

Synchronisation of moving pictures and radio, for educational purposes, has been carried out in the schools of Cleveland, U.S.A. A total of 10,000 children listened at the same time to Dr. Wells, explorer, who talked on “Coldest Africa,” while moving pictures of the scenes being described were displayed. Dr. Wells spoke from a central studio to the different schools that were listening-in, and simultaneously copies of the film were being shown, synchronised, so that the radioreceived lecture fitted in precisely as though the speaker were explaining it at each school.