B.B.C. Engineer Summarises Knowledge on Fading

Radio Record, Volume II, Issue 6, 24 August 1928, Page 11

B.B.C. Engineer Summarises Knowledge on Fading

By Capt.

P. P.

Eckersley

Chief Engineer, British

Broadcasting Co., in " The Wireless World.’’ ee ee

With the keen interest that is at present being taken in the problem of fading, the following explanations of observed phenomena, by 2a wireless engineer, who by virtue of his duties is faced with difficulties attributable to this cause, should prove of great help to . the experimenter who is endeavouring to investigate the subject.

mF ing’ has been known to ree. "Wireless -engineers ‘for Hus some time, but the advent fe! of- broadcasting has = brought the subject into great prominence. : ‘There may be those so fortunately situated in relation to one of the broadcasting : stations who have never experienced fading, 80, at the risk of redundancy, I will try and explain first what I mean by the term, You are listening to a station 150 miles away. when all at once the signals go dead weak. You fly to the reaction handle, but. everything you do has no effect, when suddenly without warning the sounds burst out again. The number of people who have conscientiously soldered, tightened and tuned, and scratched a bewildered head, must be légion, as the number who write in, having satisfied themselves that their end is all right, and complain of the variability of the transmissions, is certainly considerable.

As a matter of fact, the transmissions by the British Broadcasting Co. are not variable, and, except where light and shade are desirable in musical items, radiation and modulation are maintained sensibly constant. he cause of the variability lies, therefore, between the transmitting station and the receiving station. THE question is, then, what is there to influence the attenuation of the waves so markedly and so variably? Why in certain places does London fade while other stations do not? Why is fading only noticeable at night, and why should night time signals be stronger than day time ‘signals in certain places, and not in other places? Why should 2LO oniy be audible a quarter of an hour after sunset in Salamanea, Spain, and why should the Shetlands get us. pretty | uniformly, while people in the Victoria district (no! London, not B.C.) experience fading effects which are never noticed, say, in Hampstead? The answer is easy as far as I am concerned, and it simply is, I don’t know! |

UT a general theory exists which I will give you, and which probably forms 2 basis on which to build the explanations of minor variabilities.

Wirstly, in spite of all we have heard about the freedom of the air, wireless waves travel through the aether, which is the postulated medium for the transmission of all electro-magnetic waves. This medium is not in any sense of the word matter, in as much as matter is ponderable and can be analysed, weighed’ felt and experienced by the human senses, as it were. The aether is perfectly non-conducting to electricity, and to our senses it is nothing. But floating about in the aether are minute particles which in various permutations and combinations form matter-air, water, earth, and so on. Now, if matter is conductive to electricity, it impedes the progress of electro-magnetic waves travelling through the aether which holds matter. Thus, if the air which is suspended in the all-pervading aether is conductive, it impedes wireless waves. It may come as a surprise to many to know that air can be conductive; it is not usually necessary to suspend the filament terminals of your set in 4a vacuum, but air ean become quite conductive, and especially does it become so under the influence of sunlight. What happens is that the little particles called molecules in the air are made lively py the sunlight and split up into electrified units, which make possible the conduction of electricity. Thus there may be drawn a rough sketch of the world with the sun shin-

ing full on one side, leaving the other in shadow. On the sunny side there occur electrified particles. On the dark or night side these particles recombine near the earth, while many others rise up to a height and are ail huddled up together to form a sort of electrified layer, some 20 or 30 miles above the earth’s surface. Daylight diffuses the layer which at night time forms above the earth. The Jayer was first postulated by Heaviside, and is often known as the "Heaviside Layer." Near the sunrise or sunset region the diffusion is very great, owing to the sunlight being oblique to the air, and gradually towards the night side the air is cleared of particles, while towards the light side uniform diffusion sets in.

Now see what happens between two stations A and B on the night side. Some of the waves go direct, but many of the waves from A to B hit the layer and are reflected from it. The reflected waves are added to the direct rays, and therefore, as the reflective qualities of the layer vary, 80 the strength of the signal at B varies. It is as though the layer were a great mirror, and that, as it turns and changes and moves uneasily in its sleep, so the signal is reflected more or less, and so fading occurs. If this theory is true, certain things could be proved as follows experimentally :--~ (1) There should be no fading in the day time, but the signal should be uniformly weaker. This is generally true. stim ms.

(2) There should be evidence of rays considerably inclined to the vertical. Tn direction-finding work the general principle of determining in the directing of the incident waves is to use 4 frame, the angle of the vertical plane of which ean be varied. When the frame is at right angles to the oncoming waves no signals are heard, but this ean only be so if the waves are arriving horizontally; any vertical component will affect the frame equally in any

position, and no minimum will be Tound. This actually happens because a simple frame at night gives no reliable bear: ings due to the presence of the vertica! component, ".

(3) Using a frame which combines go largely the direct and the vertical ray distortion should occur with speech. This is noticeable more with a frame than with a vertical aerial. (4) Fading should be more noticeable at great distances from the transmitter than near to. (5) Wading should be more noticeable over land than over sea, owing to the greater attenuation of the direct ray. This has been noticed. FFURTHER than this it is impossible to go, because obviously the whole phenomenon depends so largely upon casual happenings. Undoubtedly, though many of the freak ranges are influenced by casual electrifications forming giant reflectors just in front of the sunset, the extraordinary difference between the power required to drive a signal across the sunset or sunrise band, to that required when this electrified band is removed, is evidence of the justness of the theory, and many of the problems of East and West transmissions are bound up with the same idea, Local fading (I mentioned that people were getting fading at Victoria, London, from 210) is due to something quite different. Perhaps the telephone service or the electric light mains are influencing factors. | zane