CANNOT BE REMEDIED

Radio Record, Volume I, Issue 37, 30 March 1928, Page 4

CANNOT BE REMEDIED

----- eee TROUBLESOME FADING "MUST BE ACCEPTED LIKE THE WEATHER." An American expert authority, dealing with the question of fading, Says in a letter to the general manager or the Broadcasting Company: "Dr. Alfred N. Goldsmith has very aptly summed up the present knowledge of fading. He contrasts it to a huge picture puzzle of many thousands of irregular pieces, of which only a few parts have been successfully put together. Although many technical papers have been published with reference both to periodic and steady fading, the sum of knowledge on this subject is so little that only the most general conclusions can be drawn. It is not unfair to state that the phenomenon of fading is little less than a mystery. There is evidence that many factors contribute to it. There are day and night effects, seasonal variations, and shorter periodic variations caused by local weather conditions and temperature and humidity conditions. "The accepted theory of periodic fading, of the two and three-minute variety is that it is based upon lagging effects. Although radio waves are considered to travel at 168,000 miles a second, so rapidly that lagging can hardly be reasonably conceived, nevertheless such effects must exist. For example, a receiving station located 386 miles from a broadcast transmitter may receive the signal directly through the air aud also by reflection from the heaviside layer. These two signal compone1ts are combined in the receiving system. If the heaviside layer fluctuates, either in its altitude or in iis properties as a reflecting layer, the amount of time lag in the reflected signal as compared with the signal reveived directly, must naturally vary. The accepted theory is that the reflected signal tends to cancel out the signal received directly and that the two, three, and four-minute fading is caused by changes in the heaviside reflection effect. "Another type of fading, encountered ia short-wave transmission, is known as selective fading. The American Telephone and ‘Telegraph Company, in iis researches in connection with the transatlantic telephone, has conducted extensive inquiries into selective fad‘ing and a technical paper on this subject may be expected at any time within the next few months. ‘The way in which selective fading is observed is to transmit simultaneously five separate audio-frequencies on the carrier wave. They are received at a distant point and filtered separately so that the intensity variation at each audio-frequency may be measured. Their intensity with respect to each other varies from moment to moment and seems to follow no regular law. At one moment, the middle frequencies may be preponderantly Joud while the upper and lower are weak, and, at the next moment, the situation may be reversed. These effects have not been observed in the regular broadcasting frequencies." The. writer concludes with the remark: "In the present state of the art, I doubt very much that informa: tion along these lines can be of nay great value to yoll. However, if any progress is made in counter: acting fading conditions, by methods used in transmission, I will inform you of them promptly. In the meanwhile, fading must be accepted much as the weather is; there is little or nothing we can do about it."