Wireless Broadcast

Star (Christchurch), Issue 17573, 25 June 1925, Page 11

Wireless Broadcast

CONTRIBUTED BY

“AERIAL."

TIIE NEW HOME OF STATION' K D.K.A A radio power plant: i:~ the new home j of Station KDKA. world pioneer broarlI easting station, with its long and short wa ve!rogth, 1 ransmit tine equipment. I capable. of transmitting or relaying radio programmes to the parts |of the world. The new home, a mile : from the East Pittsburg works of the | WestiiighouFc Electric Company, which | operates the station, has housed the j company's short-wave equipment for ; several months, and recently the longi wave set was moved from the old loca- ! lion in a building in the East Pittsburg works. j As consistent transmission is possible over great distances by the use of short-wave radio, it is the. short-wave set which has been used to transmit , programmes half way round the world, | the programmes having been regularly received from Pl"«s as distant from the transmitter as New Zealand, Australia. South Africa and Evtrope. This short-wave set is user] also for relaying programmes of material interest to the other stations operated by the Westinghouse Company, WJ3Z at Springfield, KVW at Chicago and KF.KX at Hastings, Nebraska. Use of metal was eliminated whcrccvcr possible in the construction of the. new building, to reduce the possibility of radio losses to a minimum. A composition roofing is used, and all metal circuits and pipes kept as near the ground as possible. Wood poles nearly 100 feet high arc used instead of steel towers to support the various antennae. The short wave aerials are located with the idea of getting directional effects on the long distance relaying. These aerials consist of copper tubes supported vertically on the high wooden poles to the right of the building. with a small horizontal counterpoise portion at the lower ends. Only one of these several aerials is coupled to the transmitter directly. The others are situated at predetermined distances from the main aerial, and arc energised bv induction. In fact, this system of aerials act in a similar manner to a | wireless beam transmitter. That is, they simply act as reflectors, and the

major portion of the transmitted energy is set in the direction where it is wanted. Any of the non-couplcd aerials can be made inoperative bybreaking the continuity of the upper and lower sections of the copper tube. To ensure the great power necessary for the operation of KDKA, the company supplying the electrical power has established a regular sub station in the basement of the building. Ihe substation is supplied by two feeder lines from two different power circuits fed by two generating stations, thus guarding against the possibility of interruptions to the service during broadcasting hours. The building contains office, space, shop, control room, experimental room and a large apparatus room, the studios being located elsewhere. The short wave and long wave transmittets arc at opposite ends of the apparatus r«>om. In the centre, there is a 20transmitter designed to oi>rrate on approximately 4•» metre. . and to bo used largely fur unmodulated telegraph signalling. Along the stall in lierear are two large rectifier panels, caoh barked bv 150-kilowatt transformers, while an additional bank "t transformers may be switched in. The short wave transmitting: apparatus eonsists of oscillators and modulators, a rather simple looking arrangement, but, one developed bv careful experimentation extending over a period of several sears. The transmitting valves used arc of the copper anode water-cooled ly pe, each capable of handling ter. adowatts. Four of these tubes run m parallel, each tube, however, operating below its full capacity. The cooling water is provided through spualls wound coils of rubber hose. The condenser is at the rear and consists cl two large brass pots, having pvrex glass covers with smaller pots inside -the large ones, the whole being immersed in oil. Inst above the condenser is the helix, of the copper ribbon type wound on pvrex glass supports. Inc supports being mounted on a wnoc.cn frame. The grid and plate coupling condensers are just behind the tubes. \ large amount of experimental work was nci-essarv to find the proper glass to u«e in the tubes for the high power oscillators, as the glass at first used often melted when in the. radio frequency field. . , The oscillator frame is alongside the modulator frame, which holds six tubes, with capacitv for doubling the number. Air tube which becomes, defective can lie instnntlv cut out. and anolhci one switched ill. Beside the modulator frame is the bins control and meter for registering grid current and bias voltages, while other necessary appliances are found about the apparatus. The long wave set at the opposite curl of she room is similar in appearance In the set just described, hut il is tuned to a different wave-length. Ibis transmilter can be used on powers up to fittv kilowatts. The aerial for the fiOfl metre or long wave covers quite a large area. Inc aerial is supported on Baft poles, spared

out :■) ; to form a diamond-shape f'gurr. \ rage aerial leads from IV •’no nearest the building io carl, of (I nlher three, which. in turn are tie*) I* veilin' This with n similar ...tint*' poise about ten feel above von : lent. A c«ippcr lul J'u;• i .!:.• uii ' lie main .■■ -i. •i • jiii'n I- t h aerial as.! rot.., .e-noise b.gclhe Through an inductance « oil. In the basement of the building ar hvo gigantic motor generators used fo lighting the rikimrnls. I'.aeh machine i capable of delivering 800 amperes. Th big water-cooled tubes each consum a ('lament. current <>f ab.u:t ’>o ampere: and there are as many as sixteen tube in some of tile sets. The main KDK.' studio is located in a building a mill or so away and wire circuits link uj the transmit tors with the main sturli* and also other studios around the city JOTTINGS. Rumours are afloat that something is certain to happen in the. very near future with regard to broadcasting. It is certain that something will go before Parliament this session, but just what is on the cards nobody seems to know, though plenty of people seem to possess inside information which they are not supposed to pass along. Dozens of people a few months ago paid up their 30s in the expectation of having four big stations running in a few weeks’ times. Of course it was not possible to get them erected and in working order very quickly, and by all the signs, they arc still a long way oil. Meanwhile the owners of the present stations arc still carrying on more or less under difficulties. If things come to a head this month, and the aforementioned rumour says they will, a central station will be constructed. This station may or may not lie strong enough to be well heard all over the country, but there is a possibility that its programmes may be relayed to other centres and re-broadcasted from them. This idea, while not ideal, would at any rate give the ‘'crystal” listener a chance to get his 30s per year’s worth of entertainment. A radio visitor to our cilv is Air S. Gadsden. of station 3S\Y. Melbourne. Mr Gadsden hn.s become well kno*wn in Victoria and parts of New .South 'Walethrough some excellent transmissions

from his station. Resides doing a little musical transmitting, 3SAY is a ketu experimented, and owns a fin.* station. It is not of the very high power typo, but uses three five-watt tubes. He says that the most interesting part of the set is the rectifier. Em* converting 1 lie 00 cycle A.C. in:.> D.C'.. a pair of Ainrad • S'’ tubes arc used. These have been used for over two years, and have been given some very hard work, yet are still as good as new. It some of the local stations used these rectifier tubes, they would have a much, purer note, nncl a bigger current in the aerial. ! am told that the world-famous A 3RD of Melbourne uses four of them to reel.Tv up to 2500 volts at over 200 mile-amps. They seem to stand hard work and like it.

CORE ES PON HENCE. “D.Soiif h bridge. (•‘el ling praceirciiit has an abnormally high rosisiaine. and consequently tunes flntlv. Yp\ the coils von are using arc of a. good make. If your condensers are very largo you may get n, given station with coils, say, .50, 7-5, and f>o, with tho condensers near minimum. ami then hear the same .station again with coils 25. 00 and 35, with il7o tuning condensers probably much nearer the maximum position. In this ease it is quite O.K-. as you. would have simply adjusted tho receiver to the same wave length twice, Imt used a different ratio of inductance t«> capacity each time, fry using very loose coupling l.etwee.i tlm Pri. and .See. coils. Ymir diagram shows a. detector and two stage amplifier, but you do not state the I vpe, of loud speaker. There b the pi, visibility that von are getting the maxi muni volume from il. already, but if this is not, l lie ram. } recommend that you instal a power-amplifier of the push-pull fy| >i . with 201 A liil.s in addition to your present set. The tubes should have the. icorrecl grid voltage on. or your B ha I lory will suffer, and distortion will occur. U- - a separate B battery for the P.P. amplifier, or you may be troubled with squeals, etc. 1 can give no furtho • particulars with respect to charging until 1 know the size «»r battery. If it is smaller than 1.5 ampere. hour rating, a good big elect rolvto rectifier as described on May 25, a* ill serve, but if the battery is above this figure, you will need either a. Tungar oiiifit., or el>e a. vibrating' rectifier. Either I\ q • ■ can 1)0 purchased locally. Your diagram shows no grid bias on the amplifier Lubes, and for voltages as liigu si-ivy (‘oiineel, a fiat dry cell, lietween the. filament, negative and t lawit b the short or positive side of the flat cell to the filament and look for i m provcmen t. Sorry I did not get your letter in time for Inst week’s is-