CHANGING STRENGTH

Evening Post, Volume CXVII, Issue 145, 21 June 1934, Page 21

CHANGING STRENGTH

A DECEPTIVE PHENOMENON

There is a ,strong impression among ■i-aclio listeners that' variations, especially from night to niglit, in the "power" of broadcast reception are due to changes in the voltage of the household supply from which the receiver is operated. That there are sometimes quite noteworthy changes in the volume obtained even from the local stations is a matter of common knowledge: but these must be looked for elsewhere than m the household supply. The voltage of the regular mains supply does change from time to time, it, is true, but it is only by a small percentage, even under bad conditions. The effect of such changes is shown in the heating of the valves, and in the high-tension supply to the plates. As to the valve-heaters, a change of small percentage will not appreciably affect the operation of the set. Valves are made to allow a" matgin on each side of the recommended voltage. The plate supply voltage will also change in about-the samo proportion; and for all practical purposes the difference in sound output due to 'a .minor change in plate voltage is. not appreciable by the oar, especially ' when an interval elapses ■ between the sourid outputs that are compared. ! _ If a local-station suddenly changes in volume (without the receiver being touched) and the house-lights do : not fluctuate violently, the only explanations are: (1) the aerial-earth system is faulty; (2) that there is something wrong with the receiver; (3) that something has happened at the transmitter. To determine which may take'a little time. An intermittent fault in the aerial system or the set should affect all reception, and is likely to be more noticeable in listening to distant stations. If the changes can bo definitely attached to one transmitter, the culpabUity has evidently been fixed and -the- mattersisho'iild,.be brought to the attention- of, thbje^esEqnsible. ■V-i-Tt is almost useless to make 'comparisons of volume-ot^a-igiy en statipn from mght .to night. If^tjie station:is a distant one; atmospheric conditions may piay a dominant1 part in affecting signal strength. With a local station, theso conditions :^do- not apply;" But the receiver, .whteh,listening to, such a ■station, is (always in a very-, insensitive condition; that- ,isj "the controls are turned far back,, to avoid overloading It is rarely indeed that a set is tuned to, say, 2YA, without some adjustment of. the : :«ontrols,..and>few•:]istemers can be ■.certain-- that :the;control knlbs are in exactly the same- p6sition:Voitie-'--night for acceptable volume as they were the night before. Moreover, what is acceptable volume at one time may be found not so satisfactory at another; a great deal depends upon the listening conditions, and these include the mood of the listener. There is, in fact, only one reliable way to ascertain -nhethci signal strength is constant oi not (except foi sudden oh-ingcs), and that is to ut.e a specially designed m strument (a simple Jece^ver) .^vhich measures the output not bj me ins of a loud speakei tmA the ear, but bY me^ns of a mctei Not many people have tnis at their disposals lho-X ear- is a most deceptive guide *