BROADCASTING DIFFICULTIES

Radio Record, Volume I, Issue 23, 23 December 1927, Page 2

BROADCASTING DIFFICULTIES

LOSeLS THROUGH ABSORPTION, Broadcast trausmission is subject to many influences, and where listeners grumble about distortion, fading, and weak reception im some localities they ate apparently unaware that the causes of these troubles are unavoidable, These difliculties are explained in the New York ‘Radio. Broadcast’? as follows :- "When the radio waves are thrown out, or radiated, from the aerial at the broadcasting station, they tend to spread out in all directions somewhat as do the waves in a pool of water when a stone is dropped inte it. As they travel outward in ever-increasing circlés, their initial energy ig spread over a larger and larger cireumference, so that- the intensity of the waves must corresponéingly decrease. If the energy mnierely spread out in this way, none of it being lost, the wave intensity would change inversely as the distance increased. But, due to the absorption in the atmosphere, and in the ground, of a part ef the wave energy, which is thus dissipated as heat, the falling-off of wave intensity with distance is more rapid. The amounts of absorption caused by various kinds of terrain differ widely, being smallest fer transmission over the ocean, or bodies of salt water, and increasingly greater for fresh water and dry land. Since a broadcasting station is usually not surrounded on all sides by a uniform terrain, the efficiency of wave-travel in different directions is not the same. As a result, the received Wave intensity may not be the same at all points equidistant from the transmitting station. Causes of Trouble. Mountains and steel-frame Iuilding areas of large cities cause particularly heavy absorption, which may amount to almost complete suppression of the Waves, so that on the far side of such ebstructions there is sometimes an area of very low wave intensity called a "dead spot." At such places, or near places where the terrain changes abruptly, as at a coast line, the waves may be deflected somewhat from their course and be thrown across the path of another part of the waves which has not been deflected. This gives rise to wave interference patterns of the saime tature as those produced at the edges of shadows by diffraction of light. Since radio waves are millions of times longer ‘than light waves, the patterns are rclatively givar:tic and one such pattern may cover au area of a hundred square miles or more. Within the area, the wave intensities at points separated from each other by onlv a fraction of a mile miay show wide differences.