STATIC INTERFERENCE.

New Zealand Herald, Volume LXVII, Issue 20744, 11 December 1930, Page 7

STATIC INTERFERENCE.

ELIMINATION OF NOISES.

AERIAL-EARTH CIRCUITS,

Interference with perfect broadcast reception is enough to spoil the enjoyment of any listener, slates a writer in an English journal. Especially is this so when headphones aro used, as constant clicks or hum become highly irritating. It is not proposed to deal with this subject in tho usual way, by explaining how to increase tho selectivity of the receiver itself, but by showing how to arrange tho aerial-earth system in order to obtain specific results. Increasing the selectivity by using a periodic coupling, tuned aerial coupling, or any of the ordinary methods, does, of course, help to reduce unwanted "signals," but it is presumed that the reader has already experimented with these himself. It might be mentioned, however, that tuned aerial coupling, especially with loose coupling to tho secondary circuit, will often reduce appreciably tho noises resulting from outside sources other than radio. Before attempting improvements, it is best to disconnect both aerial and earth from the set terminals and to connect each in turn, noting the difference in the amount of background noise; for it must not be forgotten that background noises are often duo to a faulty grid leak or to bad or dirty connections, nor that earth currents aro often the cause of the trouble.

A good size of aerial for loud reception is about 50ft. long, with a down-lead of 25ft., (ho complete aerial from the furthest insulator to the set terminal consisting of one continuous length of wire. If a certain fnlling-off in signal strength can be afforded, however, better selectivity will be obtained by shortening, this length considerably, while shock excitation from near by transmitters will be diminished. Atmospherics cover a wide band of wave-lengths and though there are circuits designed to cut them out, they arc by 110 means perfect, for they complicate the apparatus and greatly reduce the wanted signals as well. A short aerial is advantageous. As regards "man-rnade static," which consists of noises from electrical machinery, railways, trains, dynamo charging plants, otc., the best plan is to place the aerial as much as possible at right angles to the power lines. A further remedy is to try a different earth connection, such as water-pipe instead of outside earth, or even to use a counterpoise. The latter could consist of a single wiro stretched at a height of 10ft. from the ground and directly beneath the normal aerial. It will have to bo efficiently insulated in the same way as the aeriai. An improvement 011 this is a number of wires, suspended in the same manner, spread out fan-wise and connected together at the near end. Often a fixed condenser inserted in the earth-lead will improve matters. Interference from lighting mains occasionally results from too long a lead from grid condenser to grid of the detector valve, or the core of the intervalvo transformer being in the magnetic field of the lighting wires behind the walls. In the latter case, if it is possible to earth the core or shield the transformer in any way, do so; otherwise it will be necessary to move the sot to another part of the room. Where an output transformer is used the same applies, an additional precaution being the earthing of one side of the secondary. A frame aerial is another solution, but because of its inefficiency it is riot generally to bo recommended. Improved signal strength can be obtained by "pointing" the aerial toward the desired station. Curiously enough, this means that the free end is furthest from the station, the lead-in end doing the "pointing." If this is not possible, at any rate have the aerial at right angles (broadside on) to any station that refuses to be tuned out. Fading 's, unfortunately, always with us when receiving distant stations. This is not static, of course, but is nevertheless an acute problem that many learned minds have tried to solve. There are two things that arc often taken for fading. One is the results produced by a swinging aerial. To tighten it excessively means a great strain on the wire and a great strain 011 the mast. This has beon solved by some manufacturers, who have placed insulators incorporating springs on the market. The other is the "searching" of the nextdoor owner. The trouble is most severe when one or both parties use reaction 011 the uerial. For best results, efficient high-frequency stages are needed for both parties, but by placing the aerials as far away from each other as possible, and at right angles, and by using different earths, a cure may be effected. The condenser in the earth lead, originally advocated, should result in an improvement. It is not suggested that any one of these will cure the trouble or that even all of them together will entirely eliminate unwanted signals, but they should, at little cost, make listening 111010 pleasant.