THE WIRELESS WORLD.

Otago Daily Times, Issue 20875, 15 November 1929, Page 5

THE WIRELESS WORLD.

NEWS .AND NOTES. By Magna Vox. Items of local interest arc invited by “ Magna Vos ” for publication iu this column. It is necessary that such matter should reach this office by Tuesday of each week for insertion on the following Friday. , Metres. <ZL, Hobart 535 3AR, Melbourne . 484 IYA. Dunedin / 463 2FC. Sydney 442 , 2YA, Wellington 420 4QG. Brisbane 385 3LO, Melbourne .. .. .. 371 -BL, Sydney ... 353 -A?-- ? ydQe y 3IG 2h.1. Sydney 280 IYA. Auckland .... .. 33.3, 3YA. Christchurch 306 oCL. Adelaide 395 ANSWERS TO CORRESPONDENTS. “ Four-valve,” Ranfurly.—Your trouble is probably due to the plate lead touching the glid leak, or the two running close to each other. This results in .a ■ feed-back either by actual contact or by capacity effect. Separate the leads, and run at right angles to each other if possible. A. D. P., Maori Hill.—Examine your eliminator, as the case appears to I« one of incomplete filtering. The 2mfd condensers at present across the regulating resistance should go across from the B plus terminals to B'' minus. Try the set on a B supply of known value, as there does not seem to be anything wrong with the circuit. • “ Eliminator,” Milton.—You . give no details of the set or the eliminator, and it is, therefore, difficult to suggest definitely where the trouble may lie. I am taking it for granted, of course, . that you have checked the set for dirty or loose contacts and that the A battery is O.K. First of all, have your valves tested. A fault in the circuit from the eliminator to the detector plate may be the cause. Also, the absence of'grid bias, or the partial breakdown of a by-pass condenser which throws an extra load on the eliminator may lower the detector plate voltage. * Again a wive which has deteriorated may be “the cause of the trouble. G.,M‘R., Dunedin. —Since different makes vary according to the size of tbe plates, it is. impossible to say definitely. Some condensers of the same capacity. have twice the number of plates as those of a different make but of equal capacity. A .0003 . variable condenser, double spaced, would have a maximum capacity of .00015 mfd, if all the plates were used, and it would, therefore, he quite suitable for a short-wave receiver. If half the plates were discarded, capacity would be .000075. Tins would still be satisfactory if suitable coils were made.

BIAS VOLTAGE. When using :a. valve of fairly high amplification factor, whether a screengrid .valve dr 'a triode, as a radio-fre-quency amplifier,, it is often well worth while to use no more grid bias than is enough to prevent the flow of grid current. By so doing the a.c. resistance of. the valve is kept-down to a minimum, with a consequent increase in'the amplification afforded by the stage. Since the signal voltage with which a radio frequency amplifier has to deal is only a small fraction of a volt, there is usually no need to use more" than a maximum of half a volt grid bias in order to ensure that no grid current flows even at the moments when the signals make the grid most positive. If the grid bias is increased above the necessary minimum, every extra volt of grid bias is equivalent to reducing the plate voltage by “mu” volts where “mu” is the amplification factor of the valve. With a three-element valve for which the amplification factor is 30, it is only necessary to use 00 volts to obtain the results for -which 120 volts would bc neeot&ary if one volt and a-hall of grid bias were applied. It is clearly uneconomical to pay. for an extra 30 volts of plate battery and get nothing for it. With: six-volt valves, one can practically always spare half a volt from the -fi lament battery, so that by putting a small fixed resistance in the negative filament lead the desired grid voltage is obtained. The value of the resistance is computed by dividing I by twice the filament current of the valve, so that for a 0.1 amp filament R would have to be 3 ohms.

OUTSIDE INTERFERENCE. When the receiving equipment is located very near to electrical vires, whether tramway wires, power, or lighting mains, a continuous humming noise is at times heard in the speaker; This is due {in the case of the power and lighting mains), to the field produced by the alternating current being picked up hy the aerial or even by the coils inside the. receiver. The interference, associated with trams, is usually a series of clicks very noticeable when the trams are starting up. It frequently happens that a hum is picked up through the aerial lead-in being in -the immediate proximity of the house .lighting system, and iu order to ascertain whether the interference conies from within or without the house, fuses Should be removed and the receiver switched on. 'lf the hum has then disappeared the aerial and earth leads arc probably near an electric light wire and should he removed. Such things as vacuum cleaners, sewing machine motors, electric fans, are all liable to cause interference and must be kept as far as possible away from the receiver. If the hum is still present after the position of the aerial and earth leads lias'been altered, it may be necessary to screen the receiver by having the cabinet lined with copper or brass sheet and earthing same. In those cases where interference comes from , an outside source, the following precautions must he observed. Place the aerial at right angles or as nearly at right angles as possible to the overhead wires and choose a position for the aerial Inast which is at the greatest distance from any electrical wires. If interference is still pronounced it will be necessary to erect a counterpoise instead of an earth. This is accomplished by placin a second aerial wire, insulated in a similar manner to the aerial proper, at a distance of seven feet or eight feet from the ground. This screen or counterpoise wire is placed underneath the aerial, and its lead-in end connected to the receiver earth terminal. In localities where interference is bad, the aerial must be kept as short as possible, oO feet being ample for the reception of local stations. This will not only minimise A.O. hum and static, but will also sharpen up the tuning considerably, thus reducing interference from undesired stations. Receivers employing a loop or frame aerial arc less prone to interference than those using outdoor aerials, and in addition, the loop, on account of its directional properties increases the selectivity of the receiver considerably. B Battery Eliminators arc apt to cause interference if they are placed too close to the receiver, and for this reason they should be kept at a distance of at least throe feet from the set. WIRELESS PICTURES TRANS--SUTTED. The remarkable progress that is being made in the experimental transmission of pictures was recently demonstrated by the transmission of photographs of Mr Ramsay MacDonald, Prime Minister of England, and Mr E. T. Fisk, manag-

ing director of Amalgamated "Wireless (Australasia), Ltd., from Sydney to London by wireless, utilising the Fultografdi system of picture transmission. The transmission was effected by short wave wireless through AAV.A. Experimental Station, 2ME. The reproduction of these photos in'London was considered satisfactory, according to advices received by .Amalgamated Wireless. The transmission was the first of its kind in Australia, and the greatest distance ottr which pictures have been transmitted." by wireless. The first demonstration of the Fulfograph system of transmitting pictures by wireless was given by A.W.A. in Melbourne in June last. At that time the operation of transmitting and receiving the pictures was demonstrated in the one building, though between the transmitting apparatus and the room in which the receiving equipment was intsalled, the electric current passed through an artificial line, so in effect, although the transmitter was located in Melbourne the receiver might have been situated in a town 200 miles beyond Sydney. On that occasion the reproduction of many pictures transmitted over a distance, in effect, greater than the distance between Sydney and Melbourne, was exThe same type of Fultograph transmitting apparatus was used in the Sydney-London transmission.

NATIONAL RADIO EXHIBITION. Mr Kamsay MacDonald (Prime Minister) introduced the National Radio Exhibition to the British public by means of a broadcast speech' spoken at Chequers “ Surely this is a miracle if ever there was one, ’ho said. “It would have aroused every superstitious tremor in the press of our forefathers. It is a great prospect, full of possibilities,, including the possibilities which need to be guarded against, for when a whole continent—perhaps a whole world—can be connected with one place and brought immediately under one influence, who can foretell the changes in thought and habit which will follow? The public mind, and the public taste will be subjected to an influence xif terrific import, but st is part of the joyous effort of progress not only to make the great discoveries, but also to control them, and to see that man gets the maximum of good and the minimum of evil. The fruit-of the tree of knowledge has always been a dangerous diet, and men have often suffered severely from a sudden surfeit of it. They have survived, however, and have grown in mental stature and vigour. They will continue to do so. This new power of omnipresence, continued the Premier, should conquer distance and bring the , cultures and peoples of the world together, increasing their, understanding of one another, and so helping in that peace making to which all the enlightened’ statesmanship of the world must be directed. Nations which were able to talk to one another, to listen to the same music, to be present at the same ceremonial events, would surely he more difficult to divide in emnity and distrust.

The exhibition at Olympia this year is notable for the" number of radio receivers that draw all their power from the mains, and are the last word in simplicity and eneconopy. The tendency is well illustrated in n remarkable “ kit ” sot supplied for assembly by the home constructor at a total cost of teq guineas, and imprpnrating an eliminator that enables both high and low tension batteries to be dispensed with.

Another “ kit ” set displayed, for use with batteries, can be bought with an oak cabinet for ff). Girls demonstrate the ease with which it can bo put together.

Three-yalvo sots are much in evidence, a fashionable arrangement being to make use of a screened grid valve, u detector, and a low-frequency valve. Even with out the screened grid valve, it is considered that the modern three-valve set will be sufficiently selective to offer a good choice of programmes when the 8.8.G.'s regional scheme comes into operation. A welcome reduction in the price of portable receivers is noticeable. The price of a fivo-valrcr has dropped from 28 guineas last year to IS guineas. Makers are also trying to justify the title "portable” by reducing the weight, and one set has been lightened by- seven pounds, and incorporates such advantages as a tcal'.y uuspilloWe accumulator and a high-tension battery with a longer life. Loud speakers are also obtainable at reduced prices. There are now. excellent cone speakers at £1 10s, and a first-class moving-coil loud speaker can bo bougt for £7, compared with 10 guineas last year. This year's aristocrat among sets is a fivc-valver with three screened grid stapes. Tt is claimed that a set of this type is now superseding the eight-valve super heterodyne set, and one such receiver is said to be capable of petting " anything on the air,” and has the added advantage of attractive appearance. It is priced at £35. Most of the now models shown are for domestic use, but there are also examples of the “ sports ” wireless receiver. One of the most remarkable, intended for the real enthusiast, is a single instrument constructed for every conceivable wireless use. It is a fivevalve set, for use with any aerial, that covers a wave-length range from 16 to 2000 metres. The short-wave stations of Australia, i’. is said, can be heard as strongly as Davontry SXX. The final touch has been to make it adaptable for use ns a gramophone amplifier, in , conjunction with an electrical pick-up. ■