First Aid for the Interference Victim

Radio Record, Volume III, Issue 51, 4 July 1930, Page 23

First Aid for the Interference Victim

An Efficient Device

(By

CATHODE

HP following little device is offered as a palliative for interference arising from electric signs, tramways, leaky power lines, and the like. It is also of some service in reducing static, although the fact that it also reduces signal strength, somewhat lessens its usefulness, where long distance reception is aimed at. The device is known as a resonance wave coil, and, where the receiver and batteries can be completely shielded, is of real service in reducing static. It is not usually convenient to completely shield the receiver and batteries, and,

of course, an A.C. receiver will let noise in through the power supply. Even without complete shield, however, quite a worth-while reduction in extraneous noise can be obtained with proper adjustment. One must be pre pared for a decrease in signal strength, however. A practical circuit is, shown in fig. 1. The resonance wave coil, Li, is a single layer of fine wire wound on a cylindrical former, one end being connected to the antenna, the other left open. BT is a stationary close-fitting prass or copper tube about one-third the length of Li. This tube is connected to a tuned rejector circuit L-C, and thence to ground. SR is another metal tube, which goes direct to the aerial terminal of the receiving set, and is movable over the unused portion of Li. This tube must be slit. If the guard tube BT were directly grounded, ali the signals would be removed as well as the interference. Between BT and the ground, however, a tuned rejector circuit L-C is inserted ix such a manner that all frequencies except that to which L-O is tuned go tu the ground. The device should not be regarded as an ordinary wavetrap, however. When a wave strikes the aerial the effect is to put a number of voltage peaks along the resonance wave coil if this ig properly ‘tesig.ed with a natural period well above the broadcast band. By sliding SR along the coil, a point will be reached where the signal strength is at a maximum, provided the rejector circuit is tuned to the same wavelength as the receiver. At

this point extraneous noise will be at a minimum. . If the resonance coil and rejector circuit can be shielded, so much the better, although quite good results can be obtained without this precaution. Where the mains are used for power supply, best results will not be obtained unless the transformers are provided ‘with electrostatic shields between primaries and secondaries. Unfortunately, not many manufacturers make such transformers, although at least one New Zealand firm will make them to special order. A RESONANCE wave coil for use on the broadeast band may consist of a three-inch cardboard tube twelve inches long and wound over its entire length with single cotton-covered wire of about 30 or 82 S.W.G. A layer or

two of strong paper should cover the wire to protect it, and a copper tube four inches long may then be fitted snugly over the antenna end of the coil. The sliding tube is also of copper, but this should be only about one-half to one inch wide; furthermore, the two ends should not join, a quarter-inch separation being about right. This tube should slide easily over the coil. The coil in the rejector circuit must be of low resistance. . Sixty turns or so of 20 S.W.G. enamelled wire wound on a three-inch former will make a good coil, and may be tuned by a shunted condenser of .0005 mfd, maximum capacity., ‘THE operation of a resonance wave eoil is as follows. During the initial tuning, the L-C circuit is cut out by opening the switch SW. The re ceiver is then tuned in the usual manner and the switch SW subsequently closed. ‘Tune the rejector circuit by means of the variable condenser O flor maximum signal strength and minimum noise. Then slide SR over the coil until the best point is found. SR must be Changed for every different station received although when searching for stations it may be set at any position. During periods of freedom from interference, the resonance coil and its associated apparatus may be cut out and the aerial connected direct to the receiver. A switch may be used to ‘short out the whole installation.