RADIO AND ITS RECEIVERS

Sun (Auckland), Volume I, Issue 309, 21 March 1928, Page 14

RADIO AND ITS RECEIVERS

Conducted for THE SUN by

C. M. TAYLOR, B.Sc., M.I.R.E.

THE RADIO RECORD Readers are earnestly advised to read the last issue of the Record, especially the editorial NOTES AND NEWS NEWS FROM SPAIN A tale comes from Spain concerning a mischievous announcer at RadioBarcelona, who is blessed with a knowledge of ventriloquism. The other evening he simulated the accents of a fair damsel with such grace and distinction that offers of marriage are still arriving. THE FIRST WIRELESS SERMON It is seven years since a reiigious service was first broadcast, and the anniversary of that event has just been celebrated by the church which claims pioneer honours —the Calvary Church of Pittsburgh. On January 2, 1921, the Westinghouse Co. engineers suspended before the pulpit a single microphone which had to pick up not only the preacher’s sermon, but organ music, voices from the choir and the singing of the congregation. Now the same job is carried out by eight microphones placed in different parts of the building. The number of sermons delivered by •wireless since that historic date woulji, placed or end, cover many a weary mile. A WIRELESS MEDAL A medal in honour of the science and art of wireless has been struck by the French Government, and is about to be placed on sale. It is a product of the French mint, says a Paris message. On the obverse of the medal appear the signs of the Zodiac, with Iris, as messenger of the gods, walking through the clouds. The reverse bears a tribute to •wireless in the form of a globe surrounded by stars, and encircled by a frame aerial. The inevitable zigzags, portraying wireless waves, complete the picture. VALVES IN PARALLEL It is often recommended that in the output stage of a receiver if one power valve will not handle the power, two valves in parallel should be used. This is useful where a valve of the ordinary small power class is first being overloaded, but it is not desired to incur the heavier filament current of the large power class. Two valves similar to the

one already in use may be used in parallel. It is desirable, though not essential, that the valves connected in parallel should be of the same type. At any rate the filament characteristics should be the same. The circuit is simple, the two grids are connected together and similarly with the two plates. Probably few realise what are the constants of two similar valves connected in parallel. In the first place the voltage amplification factor is not changed, but is equal to that of one of the valves. On the other hand, the A.C. resistance is halved. For a given high tension voltage and grid bias the plate current is doubled, while the filament current is also doubled. With two matched valves and a load equal to their combined A.C. resistance, twice as much power can be handled as by one of the valves having a load equal to its A.C. resistance. Thus it is seen that the use of two or more valves in parallel will enable a heavier power to be dealt with than is possible with a single valve. Where it is desired to operate loud speakers at more than usual volume the paralleling of output valves should be tried.

AWARD TO PROFESSOR FLEMING The Council of the Institution of Electrical Engineers has made the award of the seventh Faraday Medal to Professor J. A. Fleming, the inventor of the forerunner of the modern wireless valve. Professor Fleming was Professor of Electrical Engineering in the University of London from 1910 until his retirement last year. As readers are no doubt aware, he has been intimately associated with the development of all the great applications of electrical science in the last 2 5 years.

GIANT LOUD-SPEAKER SYSTEM MOUNTED IN AIRPLANE Quite recently a number of towns in America have been surprised by a tremendous voice from overhead, a voice so loud that it drowned out the very roar of the airplane carrying it. Ordinary traffic noises were swamped by the volume; it could even be heard underneath the elevated railroads, whose racket we usually think of as deafening, and the words of the speaker could at times be understood as far as five miles from a point directly underneath the plane—though the plane was 3,000 feet up and the sound was being directed downward, says a writer in “Q.S.T.” To generate such a voice that drowns out the roar of three airplane motors and makes a whole city stop to listen is naturally the work of a loud-speaker system that is at the least to be classed as unusual. Fundamentally, of course, it operates along the usual lines, beginning with a microphone in a sound-proof booth in the fuselage of the big tripe-engined Fokker FVII. plane, passing through a number of stages of vacuum tube amplifiers and then arriving at three high-power loudspeakers built into the ship and pointing downward. The details of the arrangement are quite naturally not for publication, since they are the result of much painstaking and expensive experiment in the direction of overcoming the excessively severe operating conditions, of which the chief are the vibration, noise and, finally, the relatively immense demand for audio power. There seems to be no secret, though, as to the final power. The last stage employs a quartette of Western Electric one-quarter kilowatt tubes fed at full voltage by a pair of winddriven generators delivering 500 watts each. Power for the filaments of this stage is derived from still a third winddriven generator. The preceding stages are operated partly from these same generators and partly from batteries.

One may guess Immediately that the operation of such an amplifier system is not exactly cheap, especially as there must be added in the cost of operating the Fokker plane with a crew of five. Several such are operated by an organisation called “Voice of the *Sky, Inc.” The plane, which operated for some days with the Hartford airport as a base, was engaged in advertising a. certain brand of cigarette by voice and music, and was under the care of Mr. George W. McCauley, a prominent radio amateur, operator of the amplifier equipment. The crew consisted of a pilot, a mechanic, and two entertainers.

When flying over the city of Hartford at 3000 ft. elevation, the voice of the plane reached the earth with sufficient intensity so that nobody who was outside in the half-mile strip under the plane’s line of flight could possibly have overlooked it. The intensity naturally decreased as one went farther away, but perhaps 50 per cent, of the words were still intelligible at six miles—though this is unusual. The manner of operating was to traverse the city repeatedly on lines half a mile apart, taking about an hour to cover Hartford, East Hartford and West Hartford, which have a total population of perhaps 180,000. Naturally, the time required is proportional to the area to be covered. On the day mentioned there were covered, in addition, nine other towns, totalling five and a-half hours, during most of which time the loud-speaker system was in operation. The audience that cAme within range of the 1,000-watt voice is hard to estimate, but must certainly have come very close to including the total population of all the towns named—and an audience of 300,000 isn’t so bad when one is transmitting at audio frequency, even though it is being done with the world’s most powerful loudspeakers.