Wireless Broadcast

Star (Christchurch), Issue 17014, 12 April 1923, Page 5

Wireless Broadcast

RADIO FREQUENCY. ( util quite recently, radio frequency . i:ipliUt ation had been very little used, < - p -dally in Some of its ad- '• •• 11 i.igts have been pointed out from time to time, but it n, seldom that any practical working uaui is given, in England. radio amplification lias ,l " used fairly wi-deiy, by means of tuned transformers. but this type intiouii.es many more controls. The an ragf amateur usually associates ri-dio a mpiinca tiou witli a multiplicity of con dens -rs. transformers potentiometers. etc. and cannot be bothered with them, since he wants a sot with the fewest possible adjustments so as cc be able to tune in to any signals quickly. Contrary to general opinion, l.uiio frequency amplification is possible on short waves, with almost any tube on the market, and with no more tuning instruments than are found on the tsual short-wave regenerator. In comparison with radio frequency work, the advantages are:—(l) That radio amplification increases weak and strong signals by the same ratio, and is not dependent upon the square of the applied v « ltage. :is in a detector, which repeats s*rong impulses much moie loudly than '• ea H ones; (2) It boosts signals before detection would otherwise be 'heard, no matter how much audio amplification may be applied; (3) It gives less amplification of extraneous and tube noises, which in an audio .set, would simply drown out am very weak signals. High frequency amplification at very short, wave lengths may be accomplished by either of the circuits Fig 1 and lig 2. The former shows the tubes coupled by a two winding transformer the primary of which is timed, while Fig 2 shows what amounts to a jingle •winding transformer, the same winding being common to the two valves which if couples. This winding is simply an ordinary variometer, or. instead, it’mav h? an inductance tuned with a variable condenser. The variometer method is. however, easily the best. Referring to Fig 1, it will be noticed 'then the plate circuit is turned to the same frequency as the grid circuit that the tube will oscillate, this being the setting for maximum amplification. This condition, however, as most valve operators will know, is not suitable for receiving phone, or even spark if the pure f*park note is desired. Just before jho set oscillates is the best possible position for damped wave work. The tube, may be stopped from oscillating by any of three methods. One May is to detune the transformer slightly - a second is to apply ai positive jotential to the grid of the first valve; ; nd a third is to insert a suitable resistance in the plate circuit, or else •wind the transformer with fine resistance wire. Also an iron-cored transi< rmer may be used, the iron seemingly introducing resistance into the circuit, and at the same time broadening the tuning somewhat. Iron-cored transit rmers for radio frequency work by the way need very careful design and construction if they are to work at all veil. Perhaps the best way. is to us;* loth the resistance wire transformer and potentiometer to obtain the greatamplification in an untuned cirSt ‘U greater amplification is to be j 1 ad when the transformers are tuned lw *l*° wave-length of the incoming mgoals. Introducing resistance in the windings has the effect oi broadening the resonance curve, by virtue of the ii-creased cerement, and makes the circuit assume a semi-aperiodic state and gives us fair amplification over a comparatively broad range of wave-lengths without tuning each individual signal, although the amplification tier valve is very nearly twice us much with tuned transformers as with untuned ones. There- is nothing very unusual in the make-up of fig. 1, siniplv the usual vario-coupler. or honeycombs, and the transormer T. Hie potentiometer should have a resistance of about 200 ohm*. The vario-coupler primary should have about eighty turns tapped at every ten turns, and should be 1 uned with the series condenser Cl. which may have a capacity of .000*5 mfds. A largei- one max - be used if , desired, although nothing will be gain- j ed by doing so. The secondary should have up to one hundred turns tapped at every tv. ent \ turns, and is tuned l»v means of the condenser C2. w bieh .should he of PTCS mfds. capacity. For the transformer wind thirty turns of >. . « wire on a farmex three inches, in diameter. Directly over chi.-, the secondary may be wound. Forty-five turns , of the same kind of wire may bo used, both windings to be in a single layer. When the primary of this transformer is shunted with a .000-5 emfd. condenser. the tuning range will be from 170 metres to 300 metres. Another tram former may be used to tune from 300 metres to 700 metros. Using the same size former an 1 wire, the primary should have fifty turns! and the secondary about seventy to sighty turns. No condenser is tie cos*ary across the transformer secondary as tuning the primnrv. in effect, also tunes the secondary owing t,, the verv tight coupling between the two windings. It can be readily seen that the grid of the firc-t valve may be made either positive or negative ‘according to tinposition of the slider on the stabiliser. . Taking the grid positive tends to stop the tube from . si-illating. and at the same time perhaps weakening the signal slightly. Signal strength mav fie restored by readjusting C2. The more

positive the grid is, the more stable the’ whole affair becomes. Signals, after being boosted or amplified by the first valve, are passed on to the de- ■• stor by means of the transformer T. Here rectification takes place, the out-

Contributed by “Aerial.”

j phones, or audio-frequency transformer lii an audio amplifier is used. The yircuit- will be found somewhat critical ir. adjustment, but will give good amplification on wave-lengths as low as 1&0 metres. Since the transformer has comparatively I>w resistance windings and hence a shv.rp resonance curve, u will only cover about ten or twenty meters with gooo amplification for a given adjustment of condensers Cl and C2. In tuning up a tuned buzzer or wave-meter mav lie coupled verv loose- | )y to the ground lead, until the proper ( transformer tuning has been found, i ’S- *- shows an arrangement which . has been making a great name for itj self locally lately. LI a nd L2 are the j windings of a vario-coupler. while VI ‘ - * ,re the orthodox grid and ! anode variometers. The plate vano- , meter takes the place of the transI former shown in Fig. I. ! Since vario-meters have a limited { useful tuning range, the set ir essenj Dally a short-wave one. working from ! I*so to 375 metres. By making larger I vario-meters, or putting more turns on j them, the range may be increased to ( about POO metres. The lower end of j th> scale suffers in this case, however, j tlie minimum wave-length to which such a set would tune being in tinregion of 250 metres. Ihe action of Fig. 2 is about the same as Fig. 1. tuning operations being very similar. The grid vario-meter i nlil . v used as follows:- On each hall [ oi a spherical rotor, four inches in its i largest diameter. wind twenty-five I turns of No. 22 s.w.g. A.C wire. 1 the extremities 'of which should lie soldered to the two shaft ends inside the rotor. The stator forms should be turned from square pieces of wood one inch and a quarter thick. Turning these is a rather tricky job. so that, the utmost care is necessary. When the''wood is hollowed out t<> fit tin rotor nicely, the' stator coils should be wound on a special former, and then stuck into the' prepared frames with shellac. A glance at any radio magazine will show how a vario-meter should be assemfiled. The plate vario-meter should Ir wound with No. 20 d.c.c. wire, on the same size formers as the grid variometer. This means that there will be slightlv fewer turns on the plate coil, which is necessary if the vario-meters are to tune evenly- over the whole range. 1 Ik* circuit Fig, 2 lias been successfully used by two Spr-ydon amateurs. Messrs H. Smith and V. Edwards, for some ,weeks past. During this ti.n-c several American amateur stations have been heard. at a good readable strength. The aerial used is 120 feel long, seventy feet high at the free* end, and forty feet at the lead-in end. On. the evening of April 1 Mr Smith heard several American amateur*, and others vvm-c logged the following evening - This work was done on a smaller aerial, consisting of one v ii- 90 feet long, 31 feet high at one end and 15 feet at the other. Only one valve (Moorhead S'.TO was used, but later a two-step amplifier was used. Some of the signals could bo easily read fo’ *' fe<*t fro the phones Rahlies ”). This record, for which Aerial ” can vouch, shows what can be accomplished hv means of the simple circuits shown* Who wants a super-heterodyne after this? JOTTINGS. It is a pleasure to hear the 180 metres CAY. ‘tuff on the air nowadays. Some good concerts are floating about, too. But just one thing: When a # message is finished, or you are stopping the transmitter for a while, why not sign off wi ;h your call in the pro per way? T heard the tail end of .- message fairly Q.S.A. last Sunday ai lirneon. All T got was “ bibi Ho, D..vi. ’ l don’t know who sent tin’s I ni. il he signs off. ho is likely to her iMini those who pick him up. He may be glad to know the range of hi■set Ever notice how Sydney finishes work, nearly always ... >' mv that vve may have any size aerial, won’t somebody with a' long backyard try out a “ Beveridge An-t-mnn." and gi .e us the results? There thisThie f ° r S ° me e ?P er 1 men ting along | CORRESPONDENCE. G.E.E. writes:- ; - 1 have a orv.fo’ j receiver, and vv. en lisC riing to a‘concert recently. 1 could plainly fi-ai-whistles fr -m valves. What is the onuee of this? Does a crystal oscillate?” Answer:—The carrier wave, emitted from t!ie concert transmitter, \va> probably acting on vour set as a heterodyne. creating a beat which en'a bled you to hear the continuous waves from nearby receiving sets. No; a crystal set oscillates of its own accord.

Sing on. sweet bird, close hid. and raise Those angel stairways in my brain. That climb from these low-vaulted davs To spacious sunshine far from pain. —Anon. We have a poor quality of faith if we cannot market it. if it is of no