TESTING OSCILLATION

Radio Record, Volume I, Issue 6, 26 August 1927, Page 15

TESTING OSCILLATION

Many people believe that no interference can be caused unless howls and squeaks are audible in the headset or the loudspeaker It should be realised that howling is an anudio-frequency noise due to oscillation of the most violent type. Interference can be, and in most cases is, caused long before the set reaches this stage. When even mild oscillation -is present energy is transferred from the set to the aerial; the receiver thus becomes for the time being a weak transmitter sending out waves of smalt amplitude. As these have usually a frequency very near to that of the transmission being received they heterodyne it and may set up in other receivers'in the neighbourhood a low continuous note which utterly spoiis reception. If the heterodyne produces a beat that is either above or below audio frequency no note will be heaid, but receiving sets in the vicinity will -be affected. Often there will be a tendency on their part to oscillate and certainly reception wil be blurred and distorted. The unmistakable signs of oscillation in one’s own set are these: As adjustments are made, the received signal grows very loud, and is accompanied by rushing or rustling noises; C.W. signals may be heard faintly if any are in progress on the same wave-length; any tendency that the set naturally has to noisiness becomes exaggerated-thus if faint cracklings are heard normally they will become very loud and may assume proportions of atmospherics when oscillation is present. But there is one absolutcly certain test for the presence of oscillation, This consists in touching the aerial terminal of the set with a wet finger. If there is no oscillation the result will be to cause signals to fall off, since by touching the terminal the aeriail is earthed to some extent; but there wili not be anything like a sharp click in the recéivers as the finger makes and breaks eentact. This click is certain proof that the set is in oscillation and that it is radiating Tnterference produced when the set is in this condition wil! not, of course, be so bad. as that which occurs when it actually howls, still its effects are fairly powerful, and they affect other receivers at a considerable cistance. During some experiments, carried out some time ago in conjunction with a friend who is also an enthusiast, it was found that if a microphone was inserted into the earth lead of a set oscillating just sufficiently to cause the clicks referred to telephonic transmissions could be made to a distance of at least half a mile. All amateurs then who are engaged upon broadcast reception, whether from home stations or from those oversea, should make this tést frequently, and should do everything in their power to avoid giving rise to interference by allowing their sets to oscillate even mildly. Another misconception which is faitly widespread is that interference can not be caused unless reaction is use Nothing could be further from the truth. A set employing tuned anode or tuned transformer coupling oscillates very readily unless the grid potentials are properly controlled and is capable of reradiating even thongh the reaction terminals are short circuited and the coil removed altogether. Those who use circnits of this type therefore should he particularly on the look ont for oscillation, and should not allow themselvés phe be lulled imto a false feeling of securny: * . ,