INTERFERENCE

Radio Record, Volume I, Issue 30, 10 February 1928, Page 3

INTERFERENCE

POWER LINE AND OTHER . CAUSES TRACING THAT TROUBLE. In many cases in New Zealand listenets are now experiencing serious interference from power-line leakages and uoises from yarions electrical appliances. Some power board engineers have taken up broadcast listening in| their leisure and probably they will now realise how serious a matter power line interference is to broadcast lis-teners-in. , An American investigator of manmade sources of electrical interference writes :- | "Sparks are produced in the normal operation of many types of electrical apparatus (such as motors, doorbells, buzzers, gasoline engines, X-ray apparatus, violet-ray machines, some forms of battery chargers, rural telephone ringers, heating-pad thermostats). Sparks are also sometimes produced at defective insulators, transformers, etc., of electric wire lines. Sparks usually give rise to electric waves which travel along the electric power wires and by them are radiated out and are then picked up by, radio receiving sets. The noise thus produced in a radio set may come from a disturbance which has travelled several miles along the electric power wires. ELIMINATE THE SPARK. "One remedy for such types of interference is to eliminate the spark. This is possible if the spark is an electrical leak and mot necessary to the operation of the machine in which it occurs. Many very useful electrical machines, however, depend for their operation on the making and breaking of electrical circuits while they are carrying current and whicnever this happeus a spark is produced, It is impossible to eliminate these machines, $0 that it is necessary to make the spark of such nature or so to arrange the circuits that the radio frequency current is reduced or prevented from radiating, "To prevent the radio frequency current produced by a spark from getting on to the lines connecting the sparking apparatus some form of filter circuit is necessary, A condenser (1 microfarad, more ot less) connected across the sparking points will shortcircuit a considerable amount of the radio frequency current, or a condenser connected from each side of the

line to ground will serve the same purpose. A choke coil in each side of the line in addition to the condensers connected to ground forms a simple filter circuit which should prevent frequencies in the broadcast range from getting on the line. A high inductance (choke toil) or high resistance connected in each side of the line changes the characteristics of the cirenit so as to reduce the amount of power radiated. If such a filter circuit is not effective or is impracticable, the apparatus may in some cases be surrounded by a solid metal sheet or wire screen which is thoroughly grounded. The screen should completely surround the apparatus. This may be difficult, For example, in shielding the ignition system of a gasoline engine the spark coils and all wires and other parts of the system must be enclosed in metal shields, and these must be very well grounded. "When any conuections are made to the power line, in order to avoid fire and personal injury, only apparatus that is carefully tested as to voltage and cur-rent-carrying capacity should he used and the power company should be consulted before making the installation. Additions to the power lines should be made only by qualified persons. TRACING THE SOURCE OF TROUBLE. "The first thing to do in tracing the source of tronble is to make sure that it is not in the receiving set itself. ‘The next thing is to open the electri¢ switch at the house meter; if the interfering noise js still heard in the radio set, the source is then known to be outside the house. It is then desirable to report the situation to the electric power company. Many of the companies have apparatus for the purpose of following up complaints of this kind, Usually a sensitive receiving set with a lcop aerial is used to determine the direction from which the interference noise comes, and this outfit is taken from place to place until the source is found. The location of such sources is often a very difficult and baffling undertaking. The trouble sometimes comes from a spark discharge over an insulator to ground, or hetween a pair of wires, or it may be that the wire is touching some object such as a tree, pole, guy wire, etc. Such a spark discharge is a loss of power to the operating company and a_ potential source of serious trouble, and for these reasons the company is probably more interested ‘in finding and eliminating this type of trouble than the radio listener Large leaks and sparks may often be observed at night, especially in hot weather, However, sparks which

are too small to be readily noticed may cause serious interference to radio reception. CLEAN THE COMMUTATOR. "Where D.C. motors are in operation near a radio receiving set interference is sometimes caused, especially when the brushes on the motor are sparking badly. ‘The sparking should be reduced as much as possible by cleaning the commutator and setting the brushes properly. The remaining interference is sometimes overcome by placing two -condetisers (about 2 microfarads each) in series across the power supply line and’ counecting their midpoint to a good ground systeim. "Another source of interference is the ringing machine used in rural telephone exchanges. Telephone engineers can reduce or eliminate interference by comnecting a filter between the machine | and the ringing feys."