Wireless Broadcast

Star (Christchurch), Issue 17180, 25 October 1923, Page 11

Wireless Broadcast

| Contributed by

"Aerial.”

UNIQUE TUNER COIRS. In order to cover tne broadcast waroiengtn range witii an untuned radioli'eyuemcy transxormer, it is necessary to nation its resonance peak somewn at, as otnerwiso eiticient amplification occurs omy at one certain wave iengtu. j*.very radio enthusiast knows how the volume of signals received from any one station would be decreased if the set were ajusted so that it would receive ail tno broadcasting stations at once with equal strength. This is just as true tor transformer* as it is for the entire set. Signal reception is improved if the transformer can be made sharply resonant to one particular frequency, instead of being compelled to respond to them all. It is a simple matter to build a transformer which will give tins sharp response. The trick comes in moving this resonance point over the broadcast range, so that it will respond t-o the frequency desired. This problem is solved, in an instrument known as a vario-transformer, the secondary of which is made as a special sort of variometer. When its inductance is varied, the sharply selective response of the instrument is moved through the range of the broadcast frequencies increasing selective ness and signal strength at on© and the same time. The fact that the transformer will not frequencies outside its wave length range reduces interference, just as a wave-trap in the aerial circuit would reduce it. The transformer coils are made up in a double D form. The stator coils are held in six stiff fibre plates spaced about a quarter of an inch apart. Foul similar plates rotate between these in the same manner as tho moving vanes of a variable condenser. Thi rotation varies the - inductance. The primary is part of the stator. The secondary winding includes the rotor and the remainder of the stator. A special opposed winding is to the primary coil for use with a compensating condenser. "When a small three-plate condenser is placed across the radio-frequency amplifying tube—connected with this winding, it balances out the capacitative feed back of the tube eliminating tion 3 yet retaining the amplifying properties of the tube. In this manner, the oscillations, which if not properly controlled, cannot cause any squeals, or distort the incoming signals in any way at ail. If regeneration or tion is required, it can still be obtained by using a tickler coil ©r a variometer in tho plate circut of the detector tub©. This particular vario- transformer, which is mad© by a reputable American firm, is five inches square and three inches deep and is mounted on an aluminium spider frame, which acts as a shield to stop body-capacity effects, so noticeable in most sets when tuning. • * Some time ago, on July 26 to be exact, there appeared in this column a description of a receiving set, which was designed by an American amateur, Mr L/ M. Cockaday. “Aerial” has received numerous letters concerning the receiver in question, the great majority of which speak highly of the set. Since then, however. Mr Cockaday has written a book called “ Radio Telephony for Everyone.” Many excellent receivers and transmitters are

described in it, and I can recommend it to all radio friends. I believe the book is obtainable in Christchurch. My copy was loaned to me by a. friend.

Manufacturers of audio amplifying transformers must find a good deal ot difficulty in satisfying everyone. Some experimenters like an instrument with a very high ratio between primary and secondary windings. This type' is designed to give a great volume of sound per stage. Others again, have a fancy tor the lower ratio transfomer which gives less volume, and in the majority of cases, less distortion Yet the actual difference i n volume between the two types may be negligible, because tile difference m the transformers may be the Wher ratio one has fewer 1,,™? ° n i he P rllnar y. Decreasing the number of primary turns certainly increases the winding ratio, but it also decreases the power absorbed by the transformer, and there you are. The proper method of increasing the ratio is to increase the number of secondary V™-,,. °P° a™ has now solved iti difficulties by turning out three different ratios of transformers. The low- ““ » ra * io of three to one, the hafa tef'T cno. anri the highest arv ■< to ° ne ratio between primand secondary turns. That, simply means that m the first case the transioriner secondary has three times as y turns on it as the primary. Supposing that all three instrument ha™ each the same number of primarv deW nf” th ® k’ghest ratio one will deliver the greatest volume, using tho TZ°J aIVe ’ t! ? at is assuming the thr<£ transformers have the same efficiency. H hen only one step of audio apmlific'a- , . f to h® used, and volume is re- ? e l en at t,3e €x P ense of perfect mi > u+ dU n tlo ? 1 j / he hißl * ratio instrument should be used But suppose two or three stages are intended u. work a loud speaker, then tile three to one transformer should be chosen, a little volume in each stage can easily be sacrificed, when more than two stages are used, because to get decent results with a metal-horned loud speaker pure undistorted signals should be fed into it. This is because a loudspeaker is sometimes capable of distortlon 1 . 1 . n. So. when choosing an amplifying transformer, don’t grab the first one in sight, but select one to suit your needs. There are all tvpes obtainable at a moderate cost. Operator at sea, via radio, to another a few miles away: “Say O.M. did you hear that Jap* boat 3000 miles away. Second operator: Sure, heard him at 4000 miles using a crystal.” Fed-up land station operator, butting in: “ That’s nothing, O.M. heard him hauling up his anchors before he left Japan.” Next, please. ANSWERS TO CORRESPONDENTS. Curios.—Regret to say that no information of “ G.E.F.P.’e ” installation can b© made public. The station you beam’d at the time mentioned was, I believe, 3A.F. of Christchur- h Ho was using unrectified alternating current on the plate of his transmitting valve. He sends on Tuesday aud Friday evenings from 8 p.m. till 9.30 p.m. Your receiver seems 0.K., but I think that sixteen volts is scarcely enough plate voltage for the “Ora” valve.