Wireless Notes

Wanganui Chronicle, Volume LXXXI, Issue 18821, 11 July 1923, Page 7

Wireless Notes

(By

“The Grid.”)

AMPLIFICATION.

in wireless reception work there usually comes a time when the operator desires to increase the strength of the signals he is receiving. There are obviously two methods of effecting this. The power at the transmission end may De increased or alternatively the sensitiveness of the receive may be improved. The first method is of course impossible for the receiving amateur and he is forced to consider means of effecting the second. Before the development of the thermionic valve the methods of amplification were in the main merely highly sensitive relays very similar to those used in land line telegraphy. The difference was that owing to the vibratory nature of the wireless signals the moving parts of the relays had to be very light. With the introduction of the valve, however, there came a new era in reception, because one of the properties of this wonderful instrument is that it will increase the amplitude of electrical impulses applied to it. i There have been several methods developed whereby advantage may be taken of this property of the valve. These methods may be divided into two classes, which ars usually termed radio (or high) frequency amplification, and audio (or low) frequency implification. In the first method the signals are amplified before being detected, i.e., they are increased in strength be fore being made audible. The ad vantages of this method are con sideraole for weak signals. A de tector, of whatever kind, requires a certain minimum of energy before it will operate. Accordingly if the received signals are weak they will not operate the detector unless they are increased in strength by sonw preliminary method. This is where the radio frequency valve comes in. So long as there is an impulse of some sort in the aerial it is possible by means of radio frequency amplification to step it up to a strength great enough to operate the detector. The amplification is effected by operating the first valve on the straight part of its curve, i.e., the grid potential is adjusted so that any variations in it are reproduced in the plate current without, detec tion.

The problem then is to appt, 1 these variations in the plate cur rent to the grid of the next valve, and this is where the difficulty

■ arises. Several methods have been devised but all of these have dis I advantages. The first point to b« ! noted is that the variations are oc , curring at radio frequency and consequently a transformer with a heavy iron core cannot be used. The next point to be noted is that the instrument used between the valves must either be sensitive to all frequencies equally or else be easily tunable to the desired frequency. The obvious method is to use two coils, each tuned to the desired frequency, and couple these as desired. This method works extremely well, but is very difficult to use in practice because of the many adjustments required. Consequently a compromise is made and the sensitiveness is sacrificed a little while the range of frequencies is increased by coupling the two coils tightly and tuning only one of the windings. The primary coil is connected in the plate circuit ot the first valve, and the secondary coil in the grid circuit of the second valve.

The next method of coupling radio frequency valves Is the resistance capacity method. In this a high resistance is placed in the plate circuit of the first valve, and the differences of potential occurring across the resistance are impressed on the grid of the next valve through a condenser. This method of coupling is very useful when the wave length is above 1000 metres, but is of little use below this figure. As most amateurs desire to receive the shorter wave lengths it is not proposed to discuss this method further.

The method of coupling most in favour at tue present time is known as the reactance capacity method, me action depends upon the fact that an inductance tuned by means of a capacity to a given frequency oxters an almost iminite resistance to oscillations ot that frequency. Consequently if a suitable inductance with a parallel condensor is connected in the plate circuit of the first valve and tuned to the frequency it is desired to amplify there will be large differences of potential occurring across it. These potential differences can be applied through a condenser to the grid oi the next valve, and consequently amplification effected. The circuit used is exactly the same as the resistance capacity method, except that the resistance is replaced by the inductive and parallel condenser. Difficulty is sometimes found in operating this circuit owing to the tendency of the first valve to oscillate. This may be prevented by applying a negative potential to the grid by means of a potentiometer across the "A” battery. In place ot the inductance and condenser a variometer may be used or even a variable inductance, but the parallel condenser gives much more accurate tuning and its use is advised.

Turning now to audio frequency working, we find that the difficulties are much less. The frequency of the variations it is desired to amplify is quite small in comparison with those of radio frequency, and consequently an iron-cored transformer can be Used. This simply consists of a closed core of laminated soft iron on which are wound two coils having a large number of turns each. To save space the Wire used is extremely fine. The primary coil usually has about onehalf to one-fifth of the number of turns in the secondary. The prim-

ary coil is connected, in the plate circuit of the detector valve, and the secondary coil in the grid circuit of the amplifying valve. It is usually advisable to place a small fixed condenser of about .002 m.f. across the primary winding to allow a path lor the radio frequency component of the plate current variations. When using radio frequency transformers it is often found that, rustling noises are caused. These may sometimes be prevented by earthing the cores of the transformers. It may again be stated that audio frequency is useful only when the signals are strong enough to be heard clearly with the detector. This method of amplification will not increase the range of the station, but will merely increase the strength of signals already audible. Radio frequency will, however, increase the range ot the station hr amplifying signals which previously were too weak to operate the detector. WELLINGTON MUSIC. “The Grid” visited the station on the Ford Buildings, Wellington, during the week-end, and saw the apparatus with which the concerts we receive on Mondays and Fridays are transmitted. The power used is still 15 watts, and the proposed increase will not be taking place for some time. From the reports of reception ofi the concerts from this station it is apparently- that the efficiency is high.. "The Grid” was informed that a one-valver in Invercargill regularly reports clear reception. A new station is in course of erection on the Dominion Buildings, and will work on 250 watts within the near future. Music from this station should be very loud here and crystal workers will probably be able to hear it faintly. Mr Georgetti, of Papaiti, is now receiving music from Wellington on the crystal, but his apparatus is extremely efficient, and very few crystal stations here can expect such results. TAIHAPE’S STATION. Mr Coutts. of Taihape, has now a 15-watt transmitting station in operation, and he should be heard easily by local valve workers. “The Grid” understands that he does not work to a definite schedule at present. so that no advice can be given as to times of operation. WANGANUI WIRELESS CLUB STATION. Further tests were carried out on Tuesday, 3rd Inst., and a very fine selection of records transmitted. The following is a list of some of the items: “Amour Mio,” sung by Caruso; “Guitarre,” violin solo by Heifety; “Ah for e lui,” from "La Traviata,” sung by Galli-Curci; “Drink to me only with thine eyes.” played by the Flonsaley String Quartette; aria from “Norma.” and

(Chopin’s “Nocturne,” flute solos by Amadio; "Loch Lomond,'’ sung by Barclay; "Bella the Belle ox Luu oon,” and "Saturday Night,” by Sir Harry Lauder; “Muon song,” from "The Mikado,” sung by Miss Irene Williams; valse “Erica,” saxophone solo by Rudy Wiedoeft; "Three O'clock in the Morning,” waltz played by Paul Whiteman’s orchestra, in addition to the above the Empire Day message from His Majesty King George auu Her Majesty Queen Mary was transmitted and tills item was particularly ciear. Reports ot good reception were received ffom a large number of stations in tne district, including one at Maxwelltown. The club is greatly indebted to Messrs E. E. Gilbert and Co., who supplied the records and also the gramaphone for the test. On Saturday last further tests were carried out but no endeavour was made to send a programme. The aim is not to improve the efficiency of the station so as to get greater radiation, and a consequent increased range. Members of the club are reminded of the meeting on Monday next. A lecture will be delivered by the president on the working of the club's transmission station. "Will any listener-in kindly notify Mrs Carr, of Fiitwick, Ampthil, (England), that her son is seriously ill, and request her to como to th» hospital immediately?” This message was issued on a recent night by the British Broadcasting Company from Middlesex Hospital. There is no train or telephonic communication with Flitwick, but within 10 minutes a Bedfordshire lis-tener-in had sent a motor-car to take Mrs Carr to ths hospital. She was driven in the car to Luton, boarded a train there, and arrived at the hospital a little before midnight.