THE WIRELESS WORLD

Otago Daily Times, Issue 22065, 22 September 1933, Page 2

THE WIRELESS WORLD

NEWS AND NOTES

By Magna Vox.

( Items of local interest are invited by “Magna Vox” for publication in this column. It is necessary that such matter should reach this office by Tuesday of each week for insertion on the following Friday. 2BL, Sydney.—Bss K.C., 353 metres. 2FC, Sydney.—66s K.C., 422 metres. ' 3AR, Melbourne.—62o K.C., 492 metres 3LO, Melbourne. —830 K.C., 371 metres. SCL, Adelaide.—73o K.C., 895 metres. 4QG. Brisbane.—76o K.C., 385 metres. IYA, Auckland.—B2o K.C., 365.8 metres. 2YA. Wellington.—72o K.C., 420 metres 3YA, Christchurch.-—9BO K.C., 306 metres. 4YA, Dunedin.—6so K.C., 463 metres. ANSWERS TO CORRESPONDENTS. “ Farad,” Ranfurly.—You will find it better to include the volume control potentiometer in the set itself, although in most cases there is little noticeable difference. Sometimes a trouble, such as hum or instability, may be avoided by removing the control from the pick-up end of the long leads. D. M. F., Dunedin. —You will find it rather difficult to couple such a combination successfully. The arrangement you show presents rather too low an impedance to the Valve, but it is not harmful. The choke-condenser combination is a filter, and the speaker, so far as signal voltages are concerned, is across the valve. The variation in the choke will therefore affect the filter action only. The best arrangement would be a transformer of about 1J to 1, and having a primary impedance of 6000,0 r 7000 ohms. “ Eliminator,” Oamaru. —In designing an h.t. battery eliminator for use on a.c. mains you will find it an advantage to try for an output voltage much greater than that which you will normally require, because it will be possible to make use of decoupling resistances of a fairly high value, and this will ensure complete freedom from motor-boating and similar troubles. In addition, the surplus voltage will be found extremely handy for supplying the field current needs of a movingcoil loudspeaker when desired. GERMANY’S NATIONAL SET. It has been decreed by the German Government that receiver manufacturers shall make available a receiving set at a low cost, so that there may be as large an audience as possible for State talks and other propaganda. The brief specification is that the set shall be sufficiently selective and powerful to provide loudspeaker reception from tide main longwave station and at least one regional station at any time of the day in any part of Germany. Standard designs for a two-valve a.c. or d.c. receiver and a three-valve battery set have been prepared, and 28 firms are co-operating in the production of 75,000 a.c., 15,000 d.c., and 10,000 battery sets, the price of each ! of which will be 76 marks (about £4 at par). Batteries are provided where necessary at the cost of about 7e. It is intended to increase the power of the eight German main stations to 100 kilowatts each by the end of next year. MAGNETIC SHIELDING. Many people do not realise the difference between an electrostatic shield or screen and one which is intended to keep in stray electro-magnetic effects. Most ordinary screens are, of course, of the electrostatic variety, and are intended to screen electrical effects. For practical purposes almost any thin metallic or other good conducting sheet will do for this purpose, which is the reason why aluminium is so generally employed. Some people, however, use aluminium sheet for electro-magnetic shielding, such as the screening ox a choke or transformer, or even a mains unit. In so fat as this shields any stray electrostatic effect, it is, of course, quite all right, but it is practically useless for shielding stray electro-magnetic effects. Stray electromagnetism is much more difficult to screen than the stray electrical effect. For this purpose you want a magnetic material, and this also should be much thicker than a corresponding metal sheet for electrostatic shielding. Ordinary iron sheet will serve the purpose, and, for want of anything better, even socalled “tinplate” will do. But you get a much better shielding effect by using several sheets together instead of a single one. DOOR BAFFLES. Although the idea is not a new, one, it may be pointed out that a communicating door between two rooms makes one of the most satisfactory baffles that can i be contrived for a moving-coil loudspeaker of the cone. type. On aesthetic grounds the use of a door for this purpose may not suit everyone, but with a little forethought and ingenuity it is often possible to mount the loudspeaker so that it is not unsightly: at any rate, the front of the aperture behind which the cone is mounted may be camouflaged quite successfully. The hole may be covered with metal gauze, held in position by a circular beading, the whole being painted in the same colour as the door. With regard to the back of the door, it will generally be necessary to box-in the loudspeaker. The back of the box, again, may be covered with fabric or gauze, but should not be filled in with solid material. If the door is lightly built, resonances may be largely avoided by mounting the loudspeaker on a heavy backing board before securing it in position. DECOUPLING. Decoupling is necessary on the h.f. side of an s.g. receiver. High-frequency currents must be kept out of the h.t. supply, and it is usual to connect a condenser between the “ screen ” of the s.g. valve and the “earth line.” The reactance (this is similar to resistance) of large condensers at radio frequencies is very small indeed, and a decoupling resistance is not always inserted in the screening grid h.t. lead. A resistance, however, can cause no harm, and may do a lot of good, 600 or 1000 ohms being suitable values. That a high resistance value is not required is evident from an inspection of the reactances of various sizes of condensers at different wavelengths. For example, a .01 mfd. condenser has a reactance of 110 ohms at 2000 metres and 16 ohms at 300 metres. Similarly the figures for a .1 mfd. condenser are 11 ohms and 1.6 ohms respectively. On increasing the capacity to 1 mfd., the reactances become as low as 1.1 ohms at 2000 metres and .16 ohms at 300 metres. ULTRA-SHORT-WAVE RADIO. What promises to be the most efficient known type of transmitter for ultra-short-wave radio has been constructed for the English Radio Research Board. Described as being essentially a radio valve “turned inside out.” the new transmitter, it is claimed, will have a longer life, give stronger signals, and provide a larger range of l wavelengths than known existing types. The new transmitter is designed to produce wavelengths of one metre or less, and to the casual observer its most striking feature is that the aerial literally runs straight through the centre of the valve. The filament, which usually occupies the centre, becames a ring of wires surrounding the aerial, and it is

the rapid passage of electrons back and forth between these surrounding wires some hundreds of million times a second from which the wireless frequencies are derived. An interesting feature of the transmitter is that its design was based entirely on untried theoretical considerations. A small model has now, however, fully realised expectations,- and a valve of a size big enough for transmissions up to the greatest distances possible with ultra-short-waves is shortly to be produced. The new valve should be of special interest to military authorities, with a view to short-distance communication in the field. Its chief value from this point of view lies in the relative ease with which frequencies can be changed over a considerable range, a point of great importance where secrecy is involved and many channels of communication may be desirable. Other qualities of the transmitter _ which are being studied represent its suitability for television. Measurements are being made of its ease of modulation —the process of imposing sound or “vision” frequencies on the carried wave. While this process is an essential of any form of radio communication, it presents greater difficulties in television, _ and the results of tests of this kind will be awaited with considerable interest. As is well known, one of the greatest practical problems with which television is confronted is that of securing an adequate wave-band in the present crowded state of the ether. This would be entirely overcome if ultra-short-waves were used, since a practicable wave-band is represented in their case by only a minute range of wavelengths. YOU MUST GET THIS Complete details of every Australian A and B station programmes are now being printed a clear week in advance in WIRELESS WEEKLY, the popular Radio Journal. This feature commences with this week’s issue, and show 9 full programmes from Friday, September 22, to Thursday, September 28, inclusive. . Wireless Weekly is brimful of interest, ®nd contains interesting articles on Wireless Construction. Radio, Wit and Humour, Short Wave Notes, and is crammed full of instructive reading matter' from the pens of famous people in the world of wireless.

Don’t delay—Order your Copy Now.

WIRELESS WEEKLY. PRICE, 3d. Obtainable from all Newsagents every Thursday. —Advt. " 4 SYNCHRONISED STATIONS. In Europe and North America, where the number of broadcasting stations far exceeds the available channels, it is compulsory to utilise the same wavelength for more than one station. In the United States, for example, on a frequency of 1310 kc (229 m) there are no less than 51 stations operating. These are of relatively low. power, and are distributed throughout' the country, while their operating times-are so arranged that interference is eliminated as far as possible, in which respect some of the difficulty ia removed owing to the difference in time between the east and west coasts. In countries such as England, where there is no difference of time, and where the distances are short, satisfactory working on common waves is more difficult, the proximity of Europe, with its great variety of stations and its irregular allocations of wavelengths, introducing further complications. In the circumstances there is no alternative but to adopt a system of synchronising. In 1925 four small stations were so arranged, but not very successfully. Three years later sychronisation had progressed to an extent which w'as sufficient to allow ten of the lowpower stations to work on the same wavelength. Since the introduction of a system of twin transmitters of high power, the Scottish national station near Falkirk, which has a power of 50 kilowatts, has been synchronised with the 1-kilowatt Bournemouth station, with satisfactory results. Now the 50-kilowatt twin transmitters in the West of England have been completed. One of them has taken over the wavelength previously used by a low-power station, but the other, which is to be used for the national programme in this region, was left without a wavelength, and it ha s been decided to synchronise this station with the London rational station. As the two stations in question are of equal power, viz., 50 kilowatts, and not more than 150 miles apart, it is recognised by the engineering department of the 8.8. C. and other technical authorities that the undertaking is one of considerable difficulty, and the tests which are now being carried out are being watched with considerable interest. When two high-powered units close together are worked in this manner, apart from the fact that the service area will inevitably be limited, the result is at present unknown. The equipment necessary for synchronising is necessarily delicate and complicated. Bach station, of course, must be provided with means for keeping its frequency steady within fine limits, and this consists, in the 8.8. C., of a master oscillator which takes the form of a tuning fork driven by a valve. The tuning fork vibrates at a relatively slow rate, but the alternating currents of constant frequency which it supplies are taken to frequency multipliers and doubled again and again until the required high frequency alternating current is obtained. This is then amplified and. modulated in the usual way. The fork is kept in a thermostatically controlled oven, so that the temperature is always constant, but even so, due perhaps to variations in the plate and filament currents of the valve driving the fork, there are slight changes.. It is, therefore, necessary to provide means for enabling those in charge of the station to check the frequency at frequent intervals. At one time this procedure was carried out by one of the stations acting as the master station and sending out at intervals a carrier wave, which was used at a check on the locally-generated oscillations. However, fading and interference and like phenomena affected the accuracy of the measurements. An accurate master tun-, ing fork was therefore installed at the research headquarters, aiid from it a lowfrequency checking note derived from the master fork is regularly transmitted from the long-wave national station. The note as received at the local station is then compared with the note derived from the corresponding stage of the local tuning fork equipment. If the two are exactly in step no beat note is heard, but any difference between them results in a beat, which is, of course, equal to the difference in the two frequencies. Thus, those in charge of the station can very simply check its frequency. The process has been so well developed that without difficulty the carrier waves of stations which have frequencies of well over a million may be kept not more than five cycles out of synchronism.

HERE AND THERE. A woman listening in County Donegal (Irish Free State) with a receiver for which Customs duty had not been paid wag fined £IOO. # # * Karel Capek’s “ R.U.R.,” arranged for broadcasting, was broadcast in London recently. This play, which was written for the stage, and which was recently presented in Auckland by the Little Theatre Society, shows extraordinary suitability for the medium of radio. Its dramatic effect actually is heightened because it is conveyed by mechanism to the listener. The wireless is undoubtedly an ideal setting for the tale of scientificallyproduced men. Other points emphasised this suitability, among them the effects which radio can provide for the making of the scenes so_ vividly clear in the listener’s imagination. n* A* Owing to the economic crisis a number of French firms which had signed publicity contracts with broadcasting stations have been unable to pay the piper after the programmes have been broadcast. The stations have had no alternative but to accept payment in kind, with the result that the managerial offices of some studios are now replete with magnificent carpets, tablecloths, and other trimmings. One station received a letter from a defaulter in these terms: “The state of my treasury compels me to ask you to accept 200 Yo-Yos and a number of dolls, each of which says ‘ Mamma ’ simply on being squeezed.” ■ '!« fi It seems incredible to many people that each crash of static has its origin in a flash of lightning, yet such is indeed the case. And if you are at all doubtful as to whether there are enough flashes to go round, you need only ask the meteorologists, who will tell you that at any instant about 1800 storms are in progress over the earth’s surface, and that flashes may occur at the rate of 100 per second!