Radio age (Jan-Dec 1925)

10 RADIO AGE for January, 1925 The Magazine of the Hour SLOTS /&g ; PRlMARy SecONOWV complete transformers and put all their band. best features together to produce the one he was Tne Heart of the Set after. When he finished The point of the story is this: No it, he found he could bring matter how many stages of amplification in Kansas City on a two you have, no matter how good a detector, the allurements of a high ratio. "Ten these details, but the government expert and then falls very slowly to 560, which to one! Gee, ten times as much signal who worked them out had to build 127 is the upper edge of the broadcasting strength! Me for it!" Theoretically, it can be worked out something like that. In practice, the distortion and the difficulty of control nullify the theoretical advantage. The Bureau found that a 1 to 1 ratio between the windings gave very satisfactory amplification. (As the Frenchman said: "No doubt they are right, but God knows eet ees impossible!" A ratio of 1 to 1 1-3 is the maximum. This is obtained not by additional slots and coils, but by additional turns of wire in the slots at the secondary end of the tube. Using Iron and Steel Cores '"THE core was another problem. It had been known for a long time that iron-cored transformers would give better results for some purposes than air-core transformers. The core broadens the waveband over which the transformer is efficient. It also reduces the turns of wire necessary in the coils and the capacity that is so undesirable. But the use of iron or steel cores in radio-frequency amplifying transformers had not been considered as practical until the Bureau of Standards demonstrated that it was. Transformer action depends upon the building up and collapse of lines of magnetic force about the wires in the windings. In order to get the desired increase of voltage which the iron core is capable of assisting, the core must reach the magnetic saturation point on every oscillation. In radio-frequency transformers, this means that the lines of magnetic force must penetrate and saturate the iron core a million times per second if 300-meter waves are being received. This is impossible unless the core is made up of exceedingly thin sheets. A thickness— or thinness! ■ — of three and onehalf thousandths of an inch was unsuccessful. It had to be reduced to two thousandths before success was achieved. Seventyfive of these extremely delicate sheets of metal, or one sheet folded seventy-five times, makes up a core about the size of a square lead pencil. The core was well insulated to reduce losses. It is a simple matter to describe Slots are cut into each end of the tube, the windings being laid into these slots. The tube is closely packed with the iron laminations which form the core. Windings are of fine wire, and are connected from one to the other as above indicated. foot loop, and K. C. is a long way from Washington. Dr. Rogers, inventor of submarine and underground radio devices, brought in broadcasts from England by using these transformers, long before the recent furore about hearing from the other side of the Atlantic. Part of the job has been to compare the transformer developed by the Bureau with those produced by others. The curves tell the story. Three of them gave no sound at all in the phones on waves below 300 meters. Now that the broadcasting band is going downward, some folks are out of luck. One transformer had two good humps, like a camel, one at about 250 meters and the other at 350 meters. Unfortunately, the recent conference called by Mr. Hoover did not assign wavelengths to fit the humps of radio-frequency transformers. Another type of transformer gave a fairly high and broad peak from 320 to 360 meters. Outside of that, nothing doing. Two others rose sharply out of the silence between 300 and 350 meters, then faded away. The one developed by the Bureau and used in this test begins going strong at 200 meters, is better at 300, reaches its peak at 360 The turns of No. 38 wire are wound in slots in a series of continuous but divided coils. The windings slots are at opposite ends of the tubing as shown above. M\VEL£n/a-m tuner, or audio-frequency amplifier your set contains, the first tube and the first transformer control the oscillations of the entire set. If the transformer retuses to function at the wavelength of the station you are fishing for, you will not hear it — that's all! There are many radio-frequency amplifiers on the market and the manufacturers and dealers are mostly honest folks. They are not in the habit of emphasizing the weak points of their sets because there are too many strong points to talk about. It is up to the customer to say what band of wave lengths he wants to cover and make sure that the set he is considering will do the work he wants done. It is perfectly fair to ask for a demonstration before purchasing, or to purchase on approval. One big advantage of the type of radio-frequency amplifier that had its beginning in the Bureau of Standards is that it is a plug-in proposition. The waveband of a receiver can be changed by taking out one transformer and plugging in another, as easily as the electric reading lamp on the library table can be plugged into the socket on the baseboard. By using the proper transformers, four or five stages of radio-frequency can be used, with easy control and the minimum of distortion. One stage multiplies by 10,000 the energy received through the aerial and two stages multiply it by 1,000,000. Small, portable coil aerials can be used, with the advantage of their directional effects. Audiofrequency amplifiers will bring the sound up to the desired volume. A comparison between different RF transformers now in use. The heavy line indicates the best curve since it covers the broadcast band with a very satisfactory value of amplification. Two stages of amplification with UV 199s were used and all transformer curves were obtained under the same operating conditions. Mr. Perry will have another interesting article in the February RADIO AGE. Incidentally, the Editor would like to know what our readers' experiences have been with Radio Frequency Transformers. What type of transformers gives best results and widest rangeon the broadcast band ? Write us about it and we'll print the best letters.