Wireless and Weather "House of Disappearance"

Radio Record, Volume III, Issue 49, 20 June 1930, Page 2

Wireless and Weather "House of Disappearance"

N a paper: on the subject of "yeather and Wireless,’ read before the Royal Meteorological Society by Mr. R. A. Watson Wait, B.Se., F.Inst.P., A.M.1E.B., there is a section dealing with the always-present problem of reception of the "indirect" ray compared with the reception of the "direct" ray. Within the "service area" of a broadcasting station, the "direct" ray is not very strongly affected by conditions of light or dark along its path. By "serviee area" is meant that area of a broadcasting station within which the direct ray retains an energy level sufficiently high to give good signals in an average receiver. Beyond this area the "indirect" ray operates and is a much moré fickle and therefore 2 much more interesting element. In his lecture, Mr. Watt refers to the question in the following manner: "The service area is characterised by the relative constancy of signal strength given by the direct ray. If might, at first glance, appear that it could be indefinitely extended by improvement in sensitiyity of the ‘averuge receiver.’ "But, in fact, irrespective of @ wide range of variations in receiver sensitivity, it is found that outside a very limited service area lies a wide region in which signals may be reevived during daylight hours, but in which, once night has fallen, signals are sometimes very strong indeed; sometimes, om the other hand, they weaken to complete inaudibility, and violent alternations in strength may gecur within a few minutes. "Still further from the transmitter the signals may actually bé less variable than within this zone of acute fading. The whole group of phenomena may be satisfactorily explained by postulating interference effects betweén the direct ray and one or more indirect rays; when the direct and indirect rays arrive by paths of such jength as to reinforce one another at the receiver (the crest of a wave in the direct ray coinciding with a crest in the indirect ray) abnormally strong signals are heard; at times they will so completely neutralise one another {the crest of a wave in one ray filling the trough of a wave in the other) that the signal vanishes.

"The service area is that in which the direct ray is overwhelmingly stronger than the indirect; the zone of bad fading is that in which the direet and indirect rays are of comparable strength, so that opposition of phase can give almost complete neutralisation. In the outer area the direct ray is much weaker than the indirect, so that the residual fading phenomena are due to modi-. fications and interactions among the : indirect rays. themselves. "The fluctuations of the indirect ray are to be ascribed to irregularities in the upper conducting layer, to varying ionic cloudiness, if we maintain our meteorological language. In view of the limitation of effective service area for the direct ray it will be seen that the greater part of the world’s wireless communications is effected by indirect ray. . "Tn fact, we signal not by directing a wireless searchlight at the receiver,

but by lighting up the electrical cloud layer with the searchlight and letting, the receiver read: the lighting-up sig J nals. This searchlight analogy is strictly accurate for a ‘beam’ transmission, for ‘broadcast’ transmissions the process is like that by which we in Slough infer the existence of London by night, fiom the diffuse illumination of a cloud layer by the broadcast lighting of the streets."