Radio broadcast .. (1922-30)

What Causes Fading? 49 THE EARTH Showing the action of the sun's rays in producing an electrified layer sun shining full on one side, leaving the other in shadow. On the sunny side, what apparently is a swarm of flies is meant to represent electrified particles. On the dark or night side these particles have recombined near the earth, while many others have risen up to a height and are all 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 layer was first assumed to exist 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 toward the . night side the air is cleared of particles, while toward 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, so 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. j If this theory is true, certain things could be proved experimentally as follows: — (/) There should be no fading in the day .time, but the signal should be uniformly weaker. This is generally true. " (2) There should be evidence of rays considerably inclined to thf vertical. In direction-finding work, the general princi ple of determining the direction of the incident waves, is to use a frame, the angle of the vertical plane of which can be varied. When the frame is at right angles to the on-coming waves no signals are heard, but this can 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 found. This actually happens because a simple frame at night gives no reliable bearings due to the presence of the vertical component. (j) Using a frame which combines so 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 it. (Obvious from the diagram). This is noticed. (5) Fading should be more noticeable over land than over sea, owing to the greater attenuation of the direct ray. This has been noticed. Further 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,! t~ j j 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 eyidejnce rof the justness of the theory, and many of the "problems of East "arid West 'transmissions are bound up with the same idea.