THE WIRELESS WORLD.

Otago Daily Times, Issue 21021, 9 May 1930, Page 5

THE WIRELESS WORLD.

NEWS AND NOTES. ; By Magna Vox. Items of local interest are invited by“Magna Vox” for publication in this j column. It is necessary that such matter I should reach this office by Tuesday of each week for insertion on the following Friday; ' Metres. 7ZL, -Hobart .. ~ .. .. 535 3AR, Melbourne .. .. .. 484 . . 4YA,. Dunedin 463 2FC, Sydney .. .. .. 442 2YA, Wellington .. .. 420 4QG, Brisbane .. .... 385 3LO, Melbourne .... .. 371 SAB. Sydney ........ 316 SKY, Sydney’ .-. .. . . .. 280 ANSWERS TO CORRESPONDENTS. “ Aerial,” Port Chalmers.—lf at all possible, I should advise you to put up ’ a one-piece aerial. No matter how carefully you make your joints, they are sure to. have a higher resistance than the aerial wire, and they will always be liable to corrosion. If joints are unavoidable, solder and tape them. R.D.M., Dunedin.—The distortion you complain of is probably caused by your putting too much load on the valves in your amplifier. Try fitting a volume control across your pick-up, and you will- thus be able to regulate the amount of energy applied to the grid of the first amplifying ■ valve. ' The use of a controlling device would be quite satisfactory. “Henry,” Green Island.— (1) Your trouble may be due. to a fiat A battery, or, if you are using an eliminator it could be caused by a loose connection in the house switch or a faulty rectifier.'' Every • time the house switch breaks contact, the set will not stop suddenly, but will receive a certain amount of energy from the charged condensers. (2) Write to Messrs Whitcombe and Tombs, Ltd. Princes street. Dunedin. “Knock,” Dunedin.—Your trouble is due,: I’should say, to a faulty grid-leak. Its resistance may be too high—if so, try . one with a lower resistance. As a general rule, about two megohms are sufficient from , a broadcast receiver, and Jive megohms for a shortwave set. Sometimes, the contact in the grid . leak becomes intermittent. Should this occur, check with a Hew one. RESEARCH SHIP’S INSTALLATION. The research ship ■ Discovery 11, which recently left London on a three years’ voyage of scientific exploration in the Antarctic will keep in direct communication with Britain throughout the voyage by. short and long wave transmitting and receiving equipment. A p direction-finder, and a quenched spark emergency transmitter baa also been. fitted. The shortwave transmitter is capable under favourable conditions, of world-wide communication; and to keep in touch with ships and shore stations Over distances of 1000 miles on wave lengths of 600-800 metres, a medium-wave transmitter of li kilowatts power v will be used.. The receiving apparatus will enable the explorers'to-hear all the principal long and short wave stations of the world, while for purposes of navigation the direction-finder will be of . particular value, especially in Polar regions, where the ordinary magnetic emopass is subject to considerable errors. ' NEW AUSTRIAN PLANT, Austrian listeners are perturbed at the thought that within the next few months they will find themselves within the transmitting area of the new Prague 60-kilo-watt station which, with the additional power to be given to the Brunn and Bratislava transmitters, is likely to hamper their reception of the Vienna-Rosenhuegal broadcast. A proposal is being put forward to install, a 100-kilowatt transmitter on a site some. 20 miles from the Austrian capital and to transfer the. present Elant to Salburg, where, work has already een started on the construction of a relay station. Alternatively, the present 15-kilowatt plant may.be retaiued in order to ensure a double programme service to Vienna' listeners. BETTER VALVES AND TRANSFORMERS. . A point that is often lost sight of when considering the merits of various transformers is that present-day valves are, on the whole, much better than those available a year or two ago. The result of'this is that, for. a certain standard of- performance, the transformer need not be such an expensive _ component as formerly. To explain this point more fully, let us imagine we have a receiver having a transformer-coupled stage that is fitted with a valve whose impedance is 16,000 ohms, for a magnification factor of ,10. We remove this valve and put in a modern one, having an impedance of 8000 ohms for a _ magnification factor of 10. which is quite reasonable. Immediately it is noticed that the lower notes are strengthened and, it may so happen that the higher notes are also strengthened in comparison wtb those of middle frequency. The effect of fitting a new valve in this instance is, therefore, to improve the reproduction. If ' one happened to be satsfied with the reproduction of the old arrangement it would be possible to fit a new valve having about the same anode impedance as tfie old one, but having a greater magnification factor. The overall amplification would, therefore, be increased. It is further possible, for one to fit a 'new valve_ of lower impedance, but the same magnification factor'as the‘old valve, and also to fit a transformer having 'the same ratio as the ■ one used. at present, but having less inductance. Such, a transformer would be. cheaper, because inductance is dependent upon the size of the iron core, its quality and the number of turns of wire. Lees'iron could be used in the core or fewer turns of wire, whichever happened to be the cheaper. It is not advisable to use a transformer having poor iron, however, as distortion may be produced. The distortion is produced by the way the iron behaves when it is magnetised, and it may be serious. There is also the question of magnetic satura-tion-to be considered,-but the manufacturers : usually advise purchasers of their products how much current may , safely be passed by the primary winding. AERIALS AND INTERFERENCE. ■ Construction of aerials was formerly far better-than the usual practice to-day, because, with the limited amplification then available, it was necessary to preserve .as much as possible of the initial energy picked up by, the aerial from the passing radio wave. If-the erector will remember that any leak will cause a decrease in signal strength, reception should be greatly improved (says “ Crystal," writing in the New Zealand Traveller). Bare copper, when exposed to the atmosphere, will, in .time, corrode, and this corrosion will cause, a tremendous , increase in the high-frequency resistance, and corresponding reduction in efficiency. It is, therefore, best to use. some sort of insulated wire, preferably enamelled. Insulators should be of ample length, to prevent any leaks, and should be so placed that no portion of the aerial wire itself comes in contact with any object, such as trees, roof spouting, etc. The lead-in wire is part of the aerial, and must be treated as such, and not run any old way through windows or doors, but should be led through walls in an insulating tube, such as porcelain or glass. Bare wire should not be used as a lead to the set, for, as the aerial is grounded through the coil in the set, a contact with any power wire, in or out of the set, may result in the blowing of fuses or tubes, as well as being a fire menace. The aerial wire should never be run close to and parallel with the earth wire, as this sets up a condenser effect between the two, and can cause detuning of the aerial circuit in the set, and may also result in a decrease of strength by by-passing some of the aerial energy to the earth. Hum is very often encountered where the aerial lead is run close to and parallel with the speaker or power leads, or when an unshielded portion of the set is close to a single power lead, when inductive coupling occurs. Hum rarely enters via the power wiring directly. Construction of aerials will vary with different conditions and different receivers. In general, follow out the set manufacturer's specifications as to length. Shielded lead-in wires will often prevent pick-up of re-radiated impulses of interference from power, telephone, or other wires in the building.- Such a lead-in should bo run from the receiver to the junction of the lead-in witli the horizontal portion of the aerial. The shield, over the lead-in

wire must, to be effective, be earthed to a water tap or other grounded piece of metal, and not _to the earth terminal of the receiver. A separate earth is practically essential. If a shielded lead-in is used, the aerial length must, in order to retain the same efficiency as with an frn * Da i ry i lead ’ be increased rrom I X-3 to twice its ordinary lengtl if 18 i ,a , necess ary to counteract the capa city between the lead-in and ite shield, ho any , cas ,f> a shielded lead-in should only natc fuW hea other methods fail to elimif,f0 tC -i nter / erencC- However, shielded leadwith rlniJ* 6 ne . ce . ssar y in places congested besf £7*s- win . ng > and in this case the to nl n P or t e i Ur i e m acting the aerial is rZJsui s top portion as far away as fSfrlfjT p .°'T er firing, and then run a. 77 ll ad , to tte set. The longer loDßer must be the top or Wo d l d p .? rtlon - te counteract the shield r ,ra capacity. Local interference mav oL;oiA duCed *T. as ? uch as 80 P er cent, by so thnt d tbo n ? t e ® a i d ' in and leads tho for. 6 plck ‘ u P 1° the set is from .L!°P.- P ° rtl - on -.. Ca Pacity coupling to the npmL m’ Imu of a proper aerial, is ET We m£ ere great sensitivity is not sohomf’ i I he i? ain drawback to this th r e dlrect P lck ‘ u P Horn the paiyer IDe atray impulses, which will cause noisy reception.