Curing Man-Made Interference

Radio Record, Volume IX, Issue 2, 19 July 1935, Page 54

Curing Man-Made Interference

--~- Interference-iree reception in noise-infested localities can be obtained by erecting a special aerial system along the lines indicated below.

Hike various kinds of interference "generally experienced with radio reception can be classified into two distinct. groups; (a) Natura! statie, (b) manrmade interference. The first group can be disregarded, as there has been nothing invented yet which wili eure natural static. Certainly there are many devices offered for sale that will. reduce it, and-in a way they act quite effectively, but unfortunately practic. ally all of ‘them have the same effect, which is equivalent to actually shortening the aerial. The noise level is reduced, but at the same time the signal is weakened also; practically the same effect would be obtained without the device by turning down the yolume of the set. The second group (man-made static) ean be dealt with from another asnect altogether, for it is curable. This trouble originates from faulty powerlines, transformers, tramway systems, electric lights, vacuum cleaners, motors, flashing signs, etc., and may be picked up by the set jn three different ways(a) by the aerial, (bh) through the power mains, (c) by the receiver itself. Noises picked up by or originating in the set itself are entirely due to faulty construction or design, or.to insufficient shielding of components, This is rarely the case with a modern com-mercially-built receiver, but it often oceurs with poorly-constructed sets, It is not possible to deal further with this section here, but if any reader is suspicious that his set is insufliciently shielded he should consult his dealer or serviceman, Next it must be decided whether the man-made static which is being so troublesome is heing picked up by the aerial or is coming over the power-lines, or both. A good test is to tune the set to a point where the noise is particularly bad, and then turn the volume con. trol well up. Now remove the aerial wire and attach it to the earth terminal, but do not remove the earth lead, The effect will be a reduction in noise level, but if the man-made static continues to be very severe, it is certain that at least a portion of the interference is entering the set through the power mains, and a line filter is needed. These devices are supplied complete with instructions, and are yery simply

fitted. Vhey are connected in the power cable between the set and the power plug, which should be located ag far away from the former ag possible, On the other hand, if the noise is substantially eliminated, you will know that it was being picked up by the aerial and that an aerial filter will be requireG. As a general rule, man-made static is not radiated over long distances; it is seldom that these noises can be picked up direct by an aerial oyer distances of 25ft. to 40ft. from the source of the interference. In considering this, it must pe remembered that noise will be carried over much greater distances on power or telephone lines, and also will be radiated from these conductors, Thus, the first consideration in erecting ‘a noise-proof aerial is that it must be from 30ft, to 50ft. from any probable source of interference, and also the same distance from power lines, If the latter is not possible, then an endeayour should be made te erect the aerial at right angles to the direction of the lines. At the same time, the aerial must be erected as high as possible, as the interference field lessens as the height above the source of radiation increases, and also, within limits, the higher the aerial the greater the signal strength from all stations, Both of these mean a substantial improvement in the signal-to-noise ratio, The next consideration is. the leadin, for no matter how far the aerial is away from the receiver, the lead-in must be very close to potential noise earriers, As a matter of fact, the lead-in is generally the worst offender in picking up interference. To overcome this difficulty, commercia] inter-ference-eliminating aerials are often fitted with a length of Special shielded lead-in wire consisting of a flexible inSulated cable having a metallie braid: outside the insulation. This braid acts as a shield, preventing the conductor within from picking up the man-made Static, The noise is actually conducted to earth by the shield, and so is prevented from reaching the lead-in wire and becoming mixed with the signals picked up by the flat top of the aerial. Another difficulty now presents. itself, The shielding around the lead-in is very close to the wire within over the whole.length, and the iwo actually .

constitute a condenser, By earthing the shield, a substantial portion of the signals picked up by the flat top will be by-passed to earth, and the sensitivity of the set lowered accordingly. The easiest and most effective way to eliminate this loss'is to use two special coupling units, attaching one to each end of the lead-in. These pnits are merely transformers (step-down and step-up), but to describe how they function would mean going into techni-. calities that are beyond the scope. of this article, . However, they are. the means of stopping signal loss, which ‘ is the main consideration. By installing a shielded lead-in, the signal strength may be reduced a little. as the lead-in itself is no longer being ~ used for picking up signals. To compensate for this loss, the length of the aerial may .be increased by 20 to 80 feet without affecting the selectivity of the receiver. The requirements for reception free from man-made static can now be summavised, First, the receiver should be well shielded, Secondly, noise should be prevented from entering the. receiver through the power mains by means of a power line filter. Thirdly, the aerial should be erected as far-as possible from the source of interference, and it should be as high as possible. The leadin should be shielded and provided with coupling units. When all these | have been carefully carried out, clear reception, free from man-made inter-’ ference, should be the result. ad