Radio age (Jan-Dec 1925)

RADIO AGE for January, 1925 The Mahazine of the Hour Results with Radio Frequency R. F. Amplification Best for Distance, But Fan Must Get the Right Transformer By ARMSTRONG PERRY EXPERTS agree that for DX work in radio a good radio-frequency amplifier is needed. They have to agree, because it can be and has been proven, mathematically, experimentally and in practice among radio users, that radio-frequency amplification increases the weak signals to a greater degree than the stronger ones. Naturally, the signals from distant stations are the weakest, other things being equal, therefore they are to be helped most by radio-frequency amplification. A necessary part of an efficient radiofrequency amplifier for use with wavelengths within the broadcasting band is a transformer, through which the amplifier tubes are coupled together or the last amplifier tube coupled to the detector. For longer wavelengths, resistance coupling may be used with good results, but resistance-coupled amplifiers are not usually satisfactory for wave lengths below 1,000 meters. If amplification of power were the only thing to be accomplished, the construction of a radio-frequency amplifying transformer would be comparatively simple. Six stages of amplification will make a loud speaker roar like a factory whistle. Sounds Must Be Intelligible IT will vibrate a loud speaker's diaphragm so powerfully that it will throw a stream of air strong enough to blow out a match. But all amplification when applied to currents carrying the characteristics of voice or music, causes some distortion, and the radiofrequency amplifying transformer will make sounds unintelligible unless it is constructed and operated with the greatest nicety. It distorts less than the audio-frequency amplifier but either is a difficult piece of apparatus to design and build. Perhaps the greatest problem that confronted the builder or user of radiofrequency amplifying transformers for short wavelengths was the difficulty of obtaining a transformer that would amplify equally over a wide range of wavelengths within the broadcasting band. It was not very difficult to produce one that had a very good peak somewhere. If the user, fishing for a DX station, happened to catch one whose wave corresponded to the frequency at Above is an artist's picturization showing how Radio Frequency reaches out and helps the "DX" listener to amplify weak signals which he could not gel otherwise. If the set-builder or buyer gets the right R. F. transformers, they will bring him all signals within the range for which they were constructed. which the transformer was most efficient, he had something to tell the neighbors. But he might fish in vain for a much nearer and more powerful station without ever getting it. This made radio-frequency amplification unpopular at the start, except with experimenters who appreciated what a great future it had if properly developed. Long before radio broadcasting began, P. D. Lowell, of the Radio Laboratory of the United States Bureau of Standards, started experiments on the amplification of short waves. Brent Daniel of the Bureau took up the work after Mr. Lowell had established the. fact that a <-adio-frequency transformer could be built that would cover a fairly wide wave band. He developed the transformer just at the beginning of the broadcasting era. Imitators immediately began that old game of making "something just as good." The transformers that some of them built were developed hurriedly because the radio trade was growing rapidly and the loss of a day in getting into the patent office might mean the loss of a good many dollars. The radio education and experience of some manufacturers effectively prevented their making a successful imitation or substitute. The closest study of a piece of apparatus so apparently simple but really so intricate, by even an experienced radio man, cannot reveal at once all the knowledge of the device that was acquired by the man in whose brain it was born. Capacity Effects Hinder AS stated in "The Principles Underlying Radio Communication," a book prepared by the Bureau of Standards for the Signal Corps, " . . for short wavelengths, particularly for wavelengths of less *han 300 meters, radio-frequency amplification is attended with much difficulty caused by capacity effects between different parts of the circuit." The number of coils and turns of wire in a transformer make just so many component parts for condensers, unwanted, unwelcome, but impossible to be rid of. The experimenters at the Bureau met this difficulty in clever fashion. Making use of the well-known fact that the combined capacities of condensers connected in series is less than the capacity of any one of them when operating alone, they wound both the primany and secondary coils in a number of groups. These were so spaced from each other and from the core that they formed a series of condensers with but a very small combined capacitv. Even the wires that connected coil with coil were kept well separated, to minimize the capacity between them. This increased both the amplification and the range of wavelengths over which the transformer would operate. The coupling between primary and secondary was found to be very critical. A change of a sixteenth of an inch altered the characteristics of the transformer. The spacing between the separate coils of the primary and secondary was even more critical. A difference of a thousandth of an inch between any two coils made a decided difference in the characteristics. To insure accuracy and permanency, each coil was placed in a slot machine and micrometered in a square tube of insulating material. The size of the wire in the coils was found to be important, because the finer the wire, the closer together the turns will lie and the smaller the capacity between them will be. No. 38 wire was found to be the best. Beside this some human hair looks large. The ratio of the turns of wire in the primary to those in the secondary was still another problem that had to be worked out with painstaking exactness. The radio novice is often intrigued by