THE WIRELESS WORLD

Otago Daily Times, Issue 23594, 2 September 1938, Page 2

THE WIRELESS WORLD

By Magna Vox

NEWS AND NOTES

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 in the following Friday IYA Auckland.—6so k.c., 461.2 metres 2YA Wellington.—s7o k.c., 528 metre: 2BL Sydney.—74o k.c., 405 metres 2FC Sydney.—6lo k.c., 476 metres 3AR Melbourne.—77o k.c.. 390 metres SCL Adelaide.—73o k.c.. 411 metres 4QG Brisbane,—Boo k.c.. 311 metres 3YA Christchurch. —720 k.c.. 416.4 metres 4YA Dunedin.—79o k.c., 379.5 metres 4YO Dunedin. —1140 k.c., 261 metres ANSWERS TO CORRESPONDENTS " Quest.” Lunsden.—The 60-tum coll you have wound would cover from about 140 to 320 metres, though, as you have not given the capacity of the condenser you are using, this estimate is, of course, very approximate. It would be better If you could arrange your aerial so that it is not necessary for most of it to pass over the Iron roof of your house. “ Amateur,” Stirling.—You have not stated the type of valve you are using in the first audio oiage, but you could include a tone control by connecting one side of a fixed condenser of about .05 mfd. capacity to the plate of this valve Take the other side to one of the outside terminals of a 10,000 or 15,000 ohm potentiometer. The centre terminal of the latter should be earthed and the remaining free terminal left open. A USEFUL STATION A station at Willis Island, about 300 miles east of Townsville, exists for the sole purpose of supplying information to the weather bureaux at Melbourne, Sydney, and Brisbane. These are invaluable in heralding approaching hurricanes, 24 hours’ notice of which is now assured to coastal residents and to shipping. THE “ PARSONICAL ” VOICE The question of eliminating the “ parsonical ” voice from sermons during relayed religious services was one topic discussed recently by, ministers of religion and officials of the Australian Broadcasting Service. Many complaints had been made about the lack of appeal or of interest in many sermons broadcast. No decision was reached about the voice of the preacher except that there was a tacit agreement to try to select the best “radio parson ” available by each denomination.

LENGTHENING THE LEAD-IN TUBE Some trouble is frequently experienced by allowing the lead-in from the aerial to come too close to the outside wall of the house and causing intermittent clicks when the wire is swayed against the house by the wind. A cure is to increase the length of the lead-in tube by forcing on its outer end a piece of brass tubing two feet long and running the lead-in wire through this. As long as the insulating section of the tube passes .through the wall, the added metallic section will not have any deleterious effect on reception. Another way is to push a narrow brass tube through the insulating tube and pass the leadin wire through this tube. Incidentally, if the signal strength of your reproduction has lessened during the recent weather, it is suggested that the lead-in tube be examined. The end exposed to the weather may have gathered a lot of dirt, which is an unwanted conductor of electric current. It is essential therefore that the leadin tube, or point where the lead-in is brought inside, be kept clean and free from dust. SOS MESSAGES ' SOS messages are seldom broadcast in the Daventry short-wave programmes, but an instance occurred during May of this year when the steamer Anglo-Austrahan disappeared after passing Fayal, in the Azores. When all othrr efforts to contact the missing vessel had failed, the British Broadcasting Corporation broadcast in all the Daventry transmissions a request for any information likely to throw light on her fate. Although this produced no result, a large measure of success attended messages in the home transmissions. In the first quarter of 1938 341 were put on the air, 168, or almost 50 per cent., securing responses. No fewer than 226 messages were for relatives of sick persons and 95 were for witnesses of accidents. At the request of the police five messages were broadcast, two resulting in the apprehension of criminals. USE OF MICRO-WAVES Not so many years ago the short wav z s below 200 metres were regarded as useless for broadcasting and communication purposes, but amateur experimenters achieved such remarkable results that there is little of this part of the spectrum that is not congested by broadcasting and other radio services. Many of the major companies are delving into the sphere of micro-waves, which are of exceedingly short length, and approach the characteristics of light. For these minute waves the required antenna length is about one foot, and pigmy apparatus gives remarkable results. There is claimed to be an entire absence of static, but the ignition systems of cars are very troublesome. One large broadcasting company uses a microwave transmitter for relay purposes. The transmitter, which has a range of four or five miles, is but a tube of three inches, and uses two 10-inch aerials. It has an output of twotenths of a watt, or about one-hun-dredth of that of an ordinary lighting bulb, and weighs complete, including batteries and microphone, four pounds. ABOUT AERIALS A lot of inquiries are received concerning aerials. Although with present-day sets the aerial does not play so important a part as it did in the days of the old-fashioned receiver, it is safe to say the reception of quite a number of receivers is spoiled because of bad aerial construction The first essential is insulation Everything, as far as the aerial is concerned, must be sacrificed to perfect insulation. If the aerial and the lead-in are not perfectly insulated, so that there is minimum leakage in both wet and dry weather, long-distance reception is practically impossible. Height and length are the next two factors to be considered, and which of the two is the more important is a debatable point. In country districts, away from any powerful broadcasting station, an aerial as long as 100 feet may be used, but in the city, adjacent to most of the broadcasters, 40 to 50 feet should be ample. Forty feet is a good average height. Most aerials will fall into two classes. The inverted “L" type, which is made by slinging a length of wire between two masts and bringing one end to the instruments, is the more efficient. The other type, the “T” aerial, is very similar to the inverted “L” type, except that connection is made to the middle of the wire, making two equal lengths on either side. A “T ” aerial has a wavelength of half that of an inverted “L’ aerial of the same length. Except over cleared spaces, the “T ” aerial is the more difficult to construct, but the best results are obtainable from it. Such is the nature of the “T” aerial that, unless the lead-in is taken from a point less than 3 per cent, on either side of the electrical centre, only the major section of the aerial will be effective. Over cleared ground, with a horizontal, insulated wire, the electrical and mechanical centres are almost at the same point. If part of the aerial is over a house or trees, however, the electrical and mechanical centres will probably be very far apart. This is the reason why many otherwise perfect “T” aerials do not give good results. The difficulty of determining accurately the position of the electrical centres renders it almost useless to . erect a “T ” aerial unless it is over cleared space. The vertical aerial may consist of a length of copper or brass tubing, or it may be a simple length of wire suspended from a high point, connection being made to the lower end.