What is the Real Cause of Fading?

Radio Record, Volume I, Issue 3, 5 August 1927, Unnumbered Page

What is the Real Cause of Fading?

Complaints, by broadcast listeners, of "fading" are like the poor-always with us. The novice is generally convinced that all "fading" is due to a fault at the transmitting station, while as a point. of fact, fluctuations in strength of reception are seldom occasioned by that cause, owing to the provisions embodied in the technical equipment of a broadcast station. The following article, though written some time ago, throws much light on the subject of "fading," and the general theory of the phenomenon, as expressed therein, remains the most popularly accepted hypothesis by scientists throughout the world. New Zealand listeners who complain about "fading" will find the following as instructive as it is interesting. It is from the pen of O. F. Brown, M.A., B.Sc., technical secretary to the British Radio Research Board, and was written to give British listeners some idea of the cause of "fading," apart from the effects with which all are familiar.

The effects of fading of short wave (under 600 metres) signals are too apparent to the broadcast listener to need discussion, while on the other hand little is known of the cause. In the following pages the factors determining the received signal strength are cousidered. From the earliest days of radio telegtaphy it has been known that radio signals may vary in strength quite apart from faults or lack of aujustment of the apparatus at the transmitting or receiving station. An, operator may be listening to good signals from a distant station when these may gradually decrease in strength, until sometimes they become too weak to read, and then increase again until they become Stronger than they were originally. Such phenomena are described as ‘‘fading effects," and their explanation is to be sought in the factors influencing the propagation of the wireless waves in the intervening space between the transmitter and receiver. BROADCASTS FADE. Fading effects are more commonly met with m the reception of short waves (i.e., waves of the order of 600 metres and below) received by night over long distances. ‘he increase iu the numbers receiving broadcast signals and also the increase in amateur communication over long ranges (as, for example, the reception of ° American amateur stations in this country), have brought these effects recently into great prominence. The growing interest in short wave communication is to be welcomed by all interested in the scientific aspects of wireless communication, because the study of transmission on such waves is likely to lead to the collection of yaluable data bearing on the solution of problems of the propagation of waves. the explanation of whicli are at present matters of conjecture. Fading effects are closely allied to other phenomena of wireless transmission, such as the fact that the range of a wireless station using short waves is always greater by night than by day, or the occurrence of freak ranges when Signals from a station usually inaudible at a particular spot can be heard occasionally by night. Any general hypothesis advanced to explain any one of these effects must be capable of explaining the others. ATMOSPHERIC CAUSES. The hypothesis which is generally ¢ccepted to-day is that the upper rarefied regions of the atmosphere contain a large quantity of minute dust particles charged with electricity which have bee driven out from the sun and caught in our atmosphere. Such particles have the effect of making the portions of the atmosphere in which ‘they occur semiconductors of electricity. The physical constitution of the upper atmosphere makes it probable that these particles are sorted into more or less definite layers, the lowest of which may be supposed to be about 100 kilometres from the surface of the earth At night time these layers, encircling the earth as a semi-conducting shell, may be expected to have fairly sharply defined under surfaces. Under the direct influence of sunlight, however, it is probable that the rarefied gases of the atmosphere are directl, ionised; that is, the atoms which compose them afte broken up into positive and negative particles called ions. ‘hese ions will penetrate into the atmosphere to distances considerably below the permanent semi-conducting layers already described. *

AFTER SUNSET FADING. After sunset, when the direct action of the sun is removed, the positive and. uegative ions recombine, leaving the space below the lowest permanent semiconducting layer practically free from ions, aud therefore an insulator, Several years ago a classic series of experiments carried out by Austin and Cohen led to a somewhat complicated empirical formula being evolved giving the strength of the voltage to be expected at a receiving aerial at various distances from the transmitter. Recently Professor G. N Watson has shown mathematically that, assuming wireless waves are conducted round the earth in an insulating shell between a semiconducting earth and a semi-conducting layer in the atmosphere, a formula exactly of the same form as the AustinCohen formula is obtamed, and this result lends very strong mathematical support to the supposed existence of such layers in the atmosphere. We miay now consider the effect of the ionisation on the propagation of wireless waves. When a mediuimn is a semiconductor the effect is to increase the velocity with which a wave passing through the medium travels. In thie daytime, therefore, when the under sutface of the lowest semi-conducting layer is ill-defined, the upper portions of a wave from a transmitting station will pass through an ionised portion of the atmosphere. ‘These portions of the wave will travel faster than the lower portions which travel along the earth’s surface, and the wave will therefore be tilted over or rafracted towards the earth and will be absorbed in the ground, Also, when the waves are passing over dry soil, which is not a rood conductor of electricity, the foot of the waves will be pulled backwards, so that absorption by the ground will take place more quickly than if the wave

were passing over sea water, for example There is in addition irobably an absorption and scattering of the actuai energy of the wave by the charged particles in the medium, and this absorption.is greater for short waves than for long waves ‘The general effect of these factors is to make the day range of short waves (under 600 metres) comparatively small, . NIGHT FAVOURABLE, At night the lower portions of the atmosphere are clear of ions and the surface ot the semi-conductive layers more sharply defined. ‘hus, instead of the waves entering a charged medium and being refracted, they will encounter charged layers which can act as semi-re-| flecting surfaces, just as the surface of water acts as a partial reflector for light waves As the short waves ate more easily absorbed, so, from the physical principles involved, they may be expected to be more easily reflected than Jonger waves. The layers are semitransparent to the waves, and part of the energy in the waves will be transmitted and refracted by each layer and part will be reflected. The amount of energy reflected depends on the «angle at which the waves meet the surface. If this is a large angle most of the energy will pass through the layer; as the angle gets smaller as more energy 1s reflected. By night, then, waves from a point A may be supposed to reach B by paths such as até shown in Vig.° 1. Although the layers at night gre moderately well defined, it is to be expected that the shape of the layers 1s continually varying, and the oaths by which waves may reach B from A will be affected by the changes in the surfaces of. the layers. ‘The signals received, therefore, will be by mo meats constant in strength; in other words, "fading" effects will certainly occur at night either through changes in the constitution of the semi-reflecting sur-

face or through changes in the direction of the layers, both of which factors will affect the paths taken by the waves, Tligs. 2 and 3 illustrate roughly how the variation in signal strength

may arise through alterations in the arrangement of the Jayers. Should the lay@s be bent as shown in Vig. 2, then waves travelling along paths ()° and (4%) are reffected and rerracted so as to be lost and absorbed in the upper atmosphere. The waves following path (3) which reacli B are those radiated in a more vertical direction than those. reaching Bin Fig 1, As very little energy is usually radiated vertically from an ordinary aerial the signal strength at B will be decreased Similarly from Fig. 3 it is scen that if the layers lie in the other direction the strength of signals at B will be increased. IONISED LAYERS. It thus appears that the strength of signals receiyed, especially on short waves, will be by the condition, as regards conductivity, of the portions of the atmosphere through which the waves pass. The paths which the waves take will depend on

the sharpness of the ionised layers, aud the number of charged particles in the layers from moment to moment, and even on the varying arrangement of the particles in the layers. The strength of the signals may also depend on the nature of the ground between the receiver and the trausmitter, and may be influenced by changes in its electrical conductivity produced by the presence or absence of rain. The signals may, in addition, vary through the changes in the electrical state of the lower atz»sphere. lor example, the presence of thunder clouds may cause local absorptiou or deflection of the waves. SUNSET AND SUNRISE. Tn particular, just at sunset and sunrise the changes taking place in the conductivity of the atmosphere will be very great, as the direct ionising effects of the sun will cease and begin at those times. Fading effects may, therefore, be expected to be very pronounced, and experience fully supports this expectation. The only way in which these complicated plienomena can be investigated is by carefully organised simultancous observations carried out over a long period, The British Radio Research Board established under the Department of Scientific and Industrial Research has

taken a step towards the investigation of the problem along these lines by enlisting the co-operation of amateurs through the Radio Society of Great Britain and its affiliated societies, In many districts arrangements have been, made for several obseryers to record the strength of the signals from the Broadcasting stations whose transmissions are on wave-lengths likely to show fading effects, and to forward their observations monthly to the secretary of the board for analysis. The measurement of signal strength, unfortunately, is a matter of the greatest difficulty if accurate results are desired, and it is impossible outside a fully-equipped laboratory. Isolated observations may, therefore, not always be reliable. Nevertheless, if the variation of signals are observed simultaneously by several independent observers reliable qualitative data at least are likely to be collected, which will be of great value in the elucidation of the problems of wireless transmis-sion.--Published by permission of the British Radio Research Board.