Radio Digest (Oct 1923-July 1924)

March 29, 192J, RADIO DIGEST — Illustrated What's Wrong with Your Receiving Set? Chapter V — Causes of Reception Interference By Peter J. M. Clute JT is doubtful if there is any more difficult problem for the Radio fraternity to solve at the present time than that of reception interference. This condition of affairs, while partly due to the tremendous growth in the number of transmittingstations, may also be variously credited to telegraphic code transmission (government, commercial and amateur), heterodyning between stations, atmospheric disturbances, induction from power circuits, and re-radiation from receiving sets of improper design or operated by unskilled users. On the transmitting end, the chaotic condition of the ether is obviously due to the fact that the air is being used simultaneously by thousands of naval, commercial and private telegraphic stations, spark and otherwise, in addition to hundreds of broadcasting stations. Analysis of Interference A critical analysis of the Radio interference problem discloses the following most troublesome causes: (a) Heterodyning between broadcasting stations operating on approximately the same wave length. (b) Heterodyning between a broadcasting station and a nearby oscillating receiver. (c) Heterodyning between two oscillating receiving sets in the vicinity. "(d) Interference from Radio telegraph transmitters, especially of the spark variety. (e) Re-radiation from nearby oscillating receivers. (f) Atmospheric disturbances or static discharges. (g) Induction from electric light or power systems in the neighborhood. (h) Inductive interference from defective electrical apparatus, or from machines producing high-potential electrical discharges. In the above outline, an attempt has been made to list the various sources of interference in the order of the severity of their occurrence. While spark transmitters may at times be classed as the primary interference producers, it should be remembered that heterodyning interference is practically continuous, whereas spark interference at its worst is only Intermittent in character. This sort of interference wholly or partly spoils the ■ broadcast reception for many of the fa"ns and it is no more than proper that it should be the first variety that should be considered in detail. In deliberating on the question of interference, it should not be forgotten that although the broadcasting service at present furnishes the most popular diversion for the host of Radiophans, there have long existed other phases or applications of Radio which are considered of vital importance to world commerce. Besides this commercial aspect, it should be remembered that the present advancement of Radio is due in no small degree to the excellent work of some of the Radio amateurs and experimenters. It is, therefore, quite obvious that the question be approached in an impartial and unbiased frame of mind, so that the listeners in may acquire an intimate knowledge of the causes underlying the interference occasionally encountered during reception. Heterodyning' Interference If whistles or howls are heard when the receiver is not in oscillation, the cause might be either that two broadcasting stations are heterodyning; or a broadcasting station and a nearby receiving set are heterodyning; or two oscillating receivers in the neighborhood are heterodyning. Two or more stations, operating simultaneously on approximately the same wave length, will produce an audible tone, varying in pitch at times, due to the carrier waves of the stations heterodyning each other. This is also the case when nearby stations are broadcasting at wave frequencies nearly equal to that which it is desired to receive. It is quite manifest that with hundreds of broadcasting stations on the air at the same time, it will be really difficult to receive broadcast programs and in addition attempt to tune out the numerous heterodyned whistles. Tuning out this sort of interference requires a lot of patience on the part of the operator, inasmuch as most of it comes from the distant stations. The proper choice of receiver to eliminate interference from one station while listening in to the program from another, may be based on the following general considerations: (a) The closer a receiving set is to any broadcasting station, the more highly selective it should be in order to preclude the possibility of interference from that station. (b) The greater the range over which it is desired to receive and, hence, the more sensitive the receiving equipment is, the higher must be the selectivity to prevent interference. Code Interference Another annoying source of interference may be attributed to Radio telegraph transmitters, more particularly to those of the spark type. In this connection, it must be remembered that ship-to-shore communication is still carried on at wave lengths of about 450 meters, within the assigned broadcasting range, although an order taking commercial code off 450 meters has been issued. It is not an uncommon experience to have a heavy tirade of telegraphic code dots and dashes break in and interrupt a broadcast program. Under these circumstances, the average listener in, not realizing that there are many commercial and naval stations operating on broadcasting wave lengths, invariably aims harsh criticism at the local amateurs. Amateur Transmitters Under the present regulations the air is free to amateurs using wave lengths from 150 to 200, except during the evening broadcasting period, and some amateurs have the special license to operate at wave lengths up to 220 meters. Amateurs may use either spark or tube sets, but are not privileged to broadcast music or entertainment. Owners of tube sets may transmit either International Morse code or voice, except between the hours of 8:00 and 10:30 in the evening. Fans are urged to learn the code in order to identify the various, intercepted messages, and thus locate the offenders. As tho matter now stands, the fan places the blame on the amateurs without actually knowing who is causing the interference, wn< in the majority of cases it is ship and shore stations carrying on regular commercial traffic on their alloted wave lengths. It is expected that a new allocation of wave lengths will be made for the various classes of transmitting stations, and steps should then be taken to prevent a monopoly of the air by any on them, whether they are broadcast or telegraph stations. The definite assign i of these wave lengths and the strict enforcement of regulations may be helpful factors in controlling the ether traffic and clearing the air. Re-radiation Troubles Not the least of the causes of all the Q. R. M. in the ether is the bothersome reradiation from regenerative receiver* using too much coupling between plate and grid circuits, the adjustment will become critical, with the result that the tube will finally oscillate, thus becoming a generator of high-frequency oscillations. These oscillations, radiated from the receiving antenna system, will be picked up by receivers in the ind thus create some interference in JjX reception. With ordinary C\V tube lers, the tubes must necessarily be in a state of oscillation before any current will generated. Ji* lating must b< up in th< these CW signals. Such a condition may be obtained either by the use of a separate heterodyne or by li • the amount of regeneration until the tul bat ing. In this condition. iving set becomes in effect a transmitter, the extent of whose interfere upon the degree to which such oscillations are relayed to the aerial. Oscillating Unnecessary It is absolutely m the tube circuits o during the r< tion of broadcast prograi more, clear modulated signals canno properly received when the set is in a state of oscillation. The worst offender is the inexperienced operator who throws his tubes into oscillation in an effo: pick up the carrier waves of distant stations. This practice of keeping the and plate circuits in oscillation, while the wave length of the circuits is varied quite a wide range, should be discouraged. Comparative tests have shown that the (Continued on page 12) A ^m^-^^m^m^ knockdown NEUTRODYNE RECEIVER Unassembled Model KD-50, Freed-Eisemann Neutrodyne Receiver NOW the opportunity is presented to obtain a complete set of parts, recommended by the manufacturer, to work with each other in building your Neutrodyne set. 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