RADIO INTERFERENCE

Hawke's Bay Tribune, Volume XXVI, Issue 79, 14 March 1936, Page 12

RADIO INTERFERENCE

British Investigation At the request of the Institution ot Electrical Engineers, the British Electrical Research Association undertook, some time ago, to conduct an extensive investigation involving tests and coun-try-wide experiments of the measurement of thq intensity of interference from various sources, and also the development of methods of suppression. In dealing with interference from trolley-buses, mercury are rectifiers, and lifts, preliminary work was carried out on the long and medium wavebands with a voltmeter arranged to read the audio output. Automobile ignition systems presented a different problem, as this type of disturbance is mainly confined to the short-wave lengths, including the band at present in use for television. For preliminary measurements at these frequencies a super-regenerative receiver and valve voltmeter were employed. Using the foregoing simple methods of measurement it was found possible to determine the relative merits of the various methods of suppression and the actual degree of suppression effected. It was quickly recognised, however, that information obtained in this way, whilst valuable for immediate use, would not prove sufficiently quantitative for future needs, such, for example, as the basis of any restrictive proposals, since the latter, to be effective, must define the leve] at which a given disturbance or noise must be suppressed. The desirability of arriving at measurements on an absolute basis having been agreed, the E.R.A. proceeded to design and construct a 3-HF radio noise-measuring set covering the 200-2000-metre band, sufficiently sensitive to measure a disturbing field of 1 microvolt per metre, and also a shortwave set covering the 5-80 metre band with the same sensitivity. Investigation of trolley-bus interference showed that the most efficient combination of suppression arrangements consists of the use of lowcurrent control circuit filters, together with condenser suppressors fitted at intervals on the overhead lines. This method not only gives greater suppression than the original trolley-arm chokes alone, but avoids the fitting or a considerable weight on the roof of the bus.

There is the further advantage that Ihis method is not likely to be regarded as unworkable on the grounds of expense. This research has established that radiated disturbance from a given trolley-bus system can be reduced to a, level where it will no longer 'interfere, with a local broadcasting service. Investigation of interference from ignition systems has been confined mainly to the short-wave band, extending from 00.3 megacycles. At these frequencies strong interference is experienced.

Methods of suppression have beer, devised, the more efficient being resistances of the “distributed’’ tvpe whose self-capacity is low, inserted at fir? sparking points (the plug nnd the distributor) in order to prevent high freouency currents from reaching the high-tension leads which act as radiating aerials.

Carefully conducted investigations have shown that interference from electric lifts is transmitted along the supply mnins nnd radiated by the wiring of the lift equipment, in nnr ticnlar from the unscreened trailing loads running from the car to the halt wav box situated in the lift shaft. Mains-borne lift interference can ho effectively suppressed by the use of choices and condensers in combination

Tlio foregoing survey will serve tr show Hint the ma*>r technical difficul ties associated with tlio problem oi radio interference have not only bee' overcome, but in a satisfactory manner, as the methods of suppression indicated involve very little wpertM,