THE WIRELESS WORLD.

Otago Daily Times, Issue 20591, 14 December 1928, Page 5

THE WIRELESS WORLD.

NOTES AND RECORDS. By Magna Vox. Items of local interest are invited by "Magna Vox" for publication in this column. It is necessary that such matter ihould reach this office by Tuesday of each week for insertion on the following Friday. ANSWERS TO CORRESPONDENTS. “Howler,” Ravensbourne.—l should say that your loud speaker is standing too close to the set itself, and is thus vibrating 'the filaments of the valves. This would cause the howl you complain of. “Aerial” Henley.—Certainly, there is nothing to prevent you, using a tree as an aerial support, but be careful to see that the end of the aerial is well insulated, and that it is well clear of the foliage. A good idea is to run your supporting cable over a pulley, and fix the bottom end to a strong springy branch near the foot of the tree,, so that on a windy day the spring in the branch will allow plenty of give and thus reduce the risk of the aerial snapping. J. H. 8., Dunedin.—Allowing that you adopt a fairly conventional size and method of winding, 75 feet of wire should, provide an approximately correct inductance value. This is a somewhat rough and ready rule, but you will find it very handy, and, in the event of your finding, on trial, that the desired wave-band is not adequately covered, your only trouble will be the addition or removal of one or two turns. ON THE SHORT WAVES. RFM continues to come through at excellent strength and can generally be Picked up about 10.30 or 10,45 pan. HNE and ANH k are also Bteady f and have been on the air a good deal during the past, week, a noticeable and pleasing feature in the reception of both stations be iPS_ a T n almost entire absence of static. ci IJ , “ een somewhat inconsistent of late, and at times reception of this station has been very weak. A good deal of the “ woolliness ” has gone from <LO, Nairobi, and its transmissions have been coming through clearly and steadily, although mrrred a little at times by static. Moreover, a Morse station which works over the top of 7LO, 1 r 6S a £°°d deal with reception. POLL has been doing a good deal of: testing on duplex telephony with ANE of late, apparently fairly successfully. Working on call sign XCSI, and with a wave length of 44 metres, a new snort wave station is reported bo operating from San Lazaro, a suburb of Mexico Lity. Mexican news is broadcast at 0 a.m. and 0 p.m. daily. SSW Chelmsford is now transmitting on a wave length of 35.53 metres instead of on 24 metres as previously. KEEP THE TERMINALS CLEAN. In many cases, what is thought to bo static, can be traced directly to dirty terminals on a storage battery. These arc easily, cleaned by washing with a weak solution of bailing or washing soda. The terminals should be swabbed with tins solution until effervescence ceases. The’terminals and battery top may then be wiped perfectly clean with a rag. Besides cleaning the battery, the soda wash stops corrosive action by neutralising the acid—a result which cannot be secured by ordinary washing.

CONDENSER WITH A CRYSTAL. Iu regard to the condenser to tune a crystal set, there appears to bo some doubt among amateurs whether it is necessary that it should bo .0005 mfd maximum. As a matter of fact the capacity is dependent entirely on the size of the station to bo picked up, the aerial, and the size of the coil used. If. say, a .005 mfd maximum condenser is beiiv used, and only .0002 mfd of it is utilised to tune the coil to_ the station to be received, then auy size of variable condenser which_ hasenough capacity to roach .0002 mfd will give exactly the same results as a .0005 mfd tuned dowu to the .0002 being used. The strength of the signals will be exactly the same in all cases, providing, of course, that the condensers arc of equal efficiency. THE POTENTIAMETEK.

When iisinp a potentiometer either to adjust the bias of a detector valve or an H.F. valve, it should be shunted with a condenser of about 0.1 mfd capacitv. The majority of pptentiameters have a resistance of several hundred ohms and may introduce losses sufficient to cause a slight reduction of signal strength and. sharpness of tuning unless a suitable bypass of low HF impedance is provided. Paper condensers will be found to suit the purpose quite well. Even if the resistance of the potehtiameter is too low to introduce serious losses, its inductance will almost certainly be sufficient to upset the tuning slightly when adjustments are being made with it. It is, therefore, best to have a by-pass condcnsr in any case. AMERICAN WAVELENGTHS CHANGE. In order to _ meet the intense congestion in the United States, it was planned to change the Avavelengths of nine-tenths of the broadcasting stations from Armistice Day. It was claimed that the tomporary inconvenience caused would be more than recompensed by the clearer reception. To understand the plan upon which the new allocations are based, it is necessary to know that there are 9(5 channels or highways in the broadcast spectrum between 550 and 1500 kilocycles; that is, between the wavelengths of 545 metres and 200 metres. Each of these channels is 10 kilocycles wide, the modulating l or sound frequency extending for five kilocycles on either side on the carrier wave. When two stations are radiating energy on -the same channel, or on approximately the same wavelength, a collision occurs, and a high-pitched squeal results. Listeners will remember such an occurrence in New Zealand when 3YA was being heterodyned by a Sydney variation. Of the 96 channels, six are assigned exclusively to Canada, leaving 90 to ■ the use of the 624 stations in the United States and Alaska—an average of nearly seven stations per channel. The inevitable result was that there was hardly an American station that could be heard without a heterodyning whistle. Here was the problem which faced the commission which has just completed its investigations. There were two solutions to the problem. One was to reduce the time that some of the stations were to operate, and the other was to reduce the number of stations on the air. The Radio Commission chose the former plan. Forty channels were cleared, eight in each zone, or section of the country. Onefourth of these were assigned exclusively to tllc use of single stations. Of the other three-fourths, two or more stations

are limited in time so that they do not operate simultaneously. Most of them are high-powered stations. Consequently during the night hours there should be little or no interference on nearly half the dial. The remaining channels are allocated so as to provide for a minimum of interference.—Radio Record. UNDERWATER RADIO.

Underwater communications by radio (according to a cable) are now possible between submarines, and signals were sent over a distance of two miles. In these days of spectacular long-distance transmissions, this litle interchange of messages may sound rather tame, but apart from representing an important technical advance—although underwater wireless is not altogether new—it demonstrates the wonderful adaptability of' radio. Signals have been received in some of the deepest mines, sent from aeroplanes flying at great heights, and have been made to travrse every part of the globe. And now the ocean evidently oners new difficulties. When man harnessed the ether he brought to his service one of the most powerful and least understood forces that he has ever stumbled upon. Without ether, radio or light or heat would not be posisble. It provides the conducting medium for these factors just as air provides the path for sound* Water is a good sound conducto]7) and submarines have made use o.‘ this fact by sending'and receiving messcges delicate instruments which literally - act as the human organs of the' voice and ears. This, however, is purely a , sound device, and has no relation to wireless. , The most fascinating attribute of ether is the fact that it pervades everything, although “ ether waves '' do not pass

through all substances with equal readiness. .That is why buildings' and mountain ranges affect reception to such an extent, being semi-opaque, as it were, to the. impulses, just as most substances are opaque to light waves, which are the same kind of ether pulsations, only considerably shorter. Wireless waves are subject also to refraction, like light waves, whilst in reflection, of the transmission from that upper strata or air—called the Heaviside layer—we have an instance of the laws of reflection in radio. Water is one of the substances in which the vibrations of the length used in most radio work, move reluctantly—it is semi-opaque to them. This naturally has always rendered underwater radio operation difficult, and only, by the expenditure of enormous energy on long waves has it been possible to get signals to a submerged submarine, and then only over a very limited range.' But for practical purposes signals could not be sent from beneath the water, and previously submarines had to rise to the surface to transmit, erecting an aerial which was made collapsible for the purpose. In all probability a method has been found r.{ using short waves for underwater work, perhaps with a directional or beam system. In this way far greater distances will be covered with infinitely less power, as they have been in ordinary land working. 1 And although at present this new aspect to radio seems to carry only a warlike significance, it may lead the way to discoveries;and inventions having other purposes.