TOE WIRELESS WORLD.

Otago Daily Times, Issue 20833, 27 September 1929, Page 5

TOE WIRELESS WORLD.

NEWS AND NOTES. By Magsa Vos. « J} cma °f, local interest are invited by Magna Vox ” for - publication in this ■ c ? Ul ? in " ' s necessary that such matter should reach this- -office, by Tuesday of each week for insertion on the following Friday. ■ . . -rrr „, ' Metre?. iZh, Hobart .. .. .. ... 535 3AR, Melbourne .. .. .. 484 4\A- Dunedin .. .. .. 463 -KO, Sydney . 442 2\A. Wellington .. .. .. 420 S; Kite.lff 3BL. Sydney .. .... .. 353 2AB. Sydney .. ..... .. 316 2Ki. Sydney .. 280 IJ.A, Auckland .. .. .. 333 5* A. Christchurch .. 306oCL, Adelaide 305 ANSWERS TO CORRESPONDENTS. 'V-> Caversham.—The whistle you , complain of is probably caused by interaction, taking place between the two coils. One coil should not be ' placed inside the other. Oscillator Gore.—Place * the transformers at right angles, and if this does not improve' oscillation, try an f i. c i 0 ' c in 6eriGS with the plate of the detector valve. Reduce the detector plate voltage, and keep the grid leads and plate leads short-and as far apart as possibles '■ Try changing the detector valve, and if, two dif- ., f event transformers are used, by, reversing the connections to the primary of one of them. Be sure that your* batteries are. O.K. , '■ A, C.,” Dunedin.—-The ordinary type of valve'and the real A.C. valve differ in that the latter is indirectly heated " that is,- the. filament is simply a heating clement and does not give any emission as do other types of valves. The surface that does this 1?, known as the cathode, and is the fifth connection of the A.C.. valve. Generally speaking, a valve of this type is necessary only for the detector, ms it is only when the valve oscillates that the hum from the mains becomes pronounced. For amplifiers. • ordinary valves with fairly heavy filaments are quite satisfactory, Mosgiel.—Examine the grid ■ connections and see if there are any open circuits, and make sure that the grid leak is in order. Also, check over ■all plate leads for leakages or. high resistances. It is possible that there may be some, vibration of the. valve , filaments, and it might be advisable to alter the position of the speaker and mount the set on a spring foundation, your earth, leads, aerial, etc., w.eli ‘ away from all power circuits? .Gheck , for,audio frequency regeneration ,by .inserting phones or speaker.in plate circuit of the■ first' a.f. valve, removing the other. . . .. . A RECORD TRANSMISSION. The new Hnddart, Parker liner Westra-lia,-now on its maiden voyage to Australia, effected communication with Perth radio when the vessel was 4300 miles from Perth. This is record working for low power wireless equipment working at the commercial wavelength of 600 metres. The wireless apparatus, which is of the latest type, was manufactured in Australia and installed on the ship in Great Britain. . ■ THE SCREEN-GRID VALVE. The fact that' the screen-grid tube is the tube par excellence for radio-fre-qu.ency.'sockets,. -and that it. can be used .as a detector, coupled with the general use of single-stage.a.f. amplifiers, leads us to wonder if the a.c. screen-grid tube will not become the general-purpose tube of the future. As a- detector of the C-bias type, the 322-type tube is about three times as sensitive as, the 327-type tube, and the 324 is about' 10 times as sensitive as the familiar heater-type three-element tubes now employed in-the detector socket qf all a.c. receivers. Data now at hand indicate' that- an audio-frequency output voltage to load up a 171, or 243 type tube may be obtained with an r.f.viiiput of from 2 to 3 volts. Such a tube will have to be’lcoupled to the grid of the following tube cither through a resistance, or a choke • because of us own very high plate resistance. A 500-henry .choke or a 200,0000hm resistor, could be used, but the latter would make it-neces-sary to supply more voltage to the ground end- of the coupling resistor than is used m the power tubes—which we .shall assume will be the 245 type, for it appears to be the power tube for nearly all home receivers. Itwould then appear as though the screen-grid detector must be coupled to the following power tube through n fairly low-resistance high-inductance choke of capacity. If shunted by a 250,000-ohm resistor, and by a capacity of not over 100 rhmfd., a-500-henTy choke would’ provide a characteristic as good as , many, of the i best a.f. amplifiers now being put into commercial radio receivers. The disadvantage of-resistance or choke coupling would be, the fact that a pusnpull amplifier could not be used, and it seems to be the tendency at the present time. to make'the final stage, push-pull. If, then, another a.f.. stage were inserted between the screen-grid detector aild tllC power tube, a lot of .a.f. gain would have •to be thrown away in order to enjoy the advantage of push-pull, namely,, freedom from hum on a.c.-operation, cheapness of output transformer, and. freedom from overloading difficulties. With one or two screen-grid tubes as r.f. amplifiers a screen-grid detector, and a power tube, with pre-selecting ahead of the. r.f. amplifier, the prospective set-owner ought to have a pretty economical set. to build, operate, and keep in order. WIRELESS TELEPHONY. . A few days ago Mr Bruce, Prime Min■*tr- r i Australia, spoke by wireless telephony from-Sydney to Lord Paesfield, Secretary qf State for Dominion Affairs, at the latter s home 50-miles from Lonoou. Inis epoch-making event was the first occasion, on which a'dominion Miuister of the Crown had spoken l to a British Minister by wireless telephony. Arranged-, by Amalgamated . Wireless,, and conducted, so far as the Australian end was concerned, through the Australian de ®ip led and manufactured transmitter at iW: ? dl ° Centre, -Pcnnam Hills, and the A.W.A. receiving centre- at La Pe- ; rquag. the achievement was but one of ascries of successful oversea, telephony demonstrations by Mr E. T Fisk, manaaiL “ lr ® ct ?r of Amalgamated Wireless f l ' j Australian wireless record was established by Mr isk in /May, 1924. between .the station at Poldhu, C all. and Aaucluse, Sydney. . Further research tw A Camed - such success ( ,A'W.A. specially designed, maniifaclured, and, installed at Pennant Hills, Sydney, a 20 k.w. transmitter for overseas broadcasting and telephony—the a T St °l lte h!nd-in the oe-.tlien Heimspfiere. Towards the: end 07 1927, A.W.A. transmitted overseas the first hnnire broadcast programme.' and also the fins! world-wide broadcast programme- which were successfully received in all parts of the world,^ and were relayed in England by the British Broadcast!)!, Or ration, ihe following year .the Eucharistic Congress proceedings were transmitted oversoas by the. A;W.A. experimental station 2ME, and in addition to being rebroadcast in America, were heard b>- experimenters throughout the world. Last year Mr E. T. Fisk arranged- a telephony test between Sydney and Schenec-tadv, New York, in which Mr Lawton, late Consul-general for America, aiid representatives of Sydney newspapers, took pa11. The clarity of. the - voices at i.chencctady. and the case of conversing at tlie Sydney cud./were a revelation to those taking part. Subsequently a test was arranged with Java, and another with Amsterdam, followed by the interchange of conversation with 'Berlin. A unique example of the effective use to which wireless telephony may be put was illustrated recently by a patient at the Royal Prince Alfred Hospital, Sydney, speaking to bis mother in London. The T °L transmitting apparatus at A.W.A. Radio Centre. Pennant Hills, and the receiving apparatus at the company’s receiving centre at La Perouse, utilised in the tests, was designed .by the company's engineers, and manufactured at its works at Sydney.

BUYING AN AC SET. Good parts am especially necessary in a.c. operated sets where, in addition to the usual components,, there are filter chokes and condensers, transformers, and voltage regulating resistances, and in this type of set'ino.re than in any other it is essential to avoid cheapness, because: of

the consequences likely to arise in the! event of a. breakdown. Another point j to be watched in the a.e. set is the.classi- j fication of the set according to the number of valves in it. This has always been a common method of rating a receiver, and it is generally assumed that, eay, an eight-valve set is superior to, say, a six-valve set. . It is equivalent to stating broadly that an eight-cylinder motor car is more powerful than a six-cylinder car without taking' into account the size of the cylinders, the revolutions, and all the other factors which enter into the question. Some manufacturers have developed the misleading habit of including in the tally of valvet the rectifying valve or valves. The rectifiers are certainly necessary, but they do not in themselves add to the selectivity, sensitiveness, or tone of the set, as their place could quite, easily be taken hy batteries of equal capacity with, perhaps, better results hi some cases. In the a.c. set hum is also a. consideration. Its absence does not always indicate perfection because the audio transformers and speaker may not be able to reproduce notes sufficiently low to include hum frequencies. In thin case the name and reputation of the maker has to be relied upon, , A .good test for hum iS'-to keep the demonstrator from tuning in only strong local stations, from which the signal is likely to drown the hum, and persuade him to tune to a weaker or more distant station and bring it up to full volume. Then detune the set when, if there is an unduly large amount of hum present, it will bo apparent without the covering effect of the signal. _ The speaker must be capable of producing low notes for tills to be a success. Hum can be reduced to such an extent that it cauuot l>e heard in a. Quiet room of medium dimensions, but to do this costs money, which explains the apparently unreasonably high price of some sets. Finally, in selecting a receiver always inquire as to what has to he done •n the way of maintenance and its pjobablo cost.