Radio Broadcast (May-Oct 1922)

346 Radio Broadcast design of practical apparatus, they however indicated that the main difficulty to be overcome was to combine considerable amplificacation with stability and that the solution of the problem became rapidly more difficult with the increase of the number of tubes used in cascade. By stabiHty, in this case, 1 mean the freedom from any sudden generation of oscillations in any part of the circuits of the amplifier. RECEIVING DEVELOPMENTS IN 1920, however, an important step was made by Mr. G. Mathieu, as to the path to be followed out in order to obtain a practical solution of the problem. This consisted in the design of a new type of air-core tuned intervalve transformer arranged in such a manner as to possess only an extremely electrostatic capacity between the windings, and having its effective primary impedance about equal to the effective internal plate to filament resistance of the tube in use when the secondary circuit was brought into resonance with the frequency of the oscillations to be amplified. The results to be achieved during the first tests of these new transformers appeared to be quite amazing, the amplification factor for one tube having passed suddenly from 5 to about 15 for the particular tube tested, whilst the stability proved incomparably better than what had been obtained previously, even when the grid of the tube was kept to a negative potential of I or 2 volts. The same principle has proved quite as successful when applied to the design of iron-core low frequency transformers. In this case, however, it was found necessary to adopt an iron magnetic shunt between the windings so as to provide a sufficiently loose coupling between the primary and secondary circuits of the transformer. Recently, Mr. Mathieu has further improved the design of his high frequency transformer by making it astatic. THE IMPORTANCE OF SHORT WAVES I SHALL now deal with another and most important branch of the science of radio telegraphy; a branch which 1 might say has been a long time most sadly neglected. It concerns the use that can be made of very short waves, especially in regard to their application to directional radio telegraphy and radio telephony. The study of short waves dates from the time of the discovery of electric waves them selves, that is, from the time of the classical experiments of Hertz and his contemporaries, for Hertz used short electric waves in all his experiments, and also made use of reflectors to prove their characteristics and to show among many other things that the waves, which he had discovered, obeyed the ordinary optical laws of reflection. As 1 have already stated, short electric waves were also the first with which ! experimented in the very early stages of wireless history, and 1 might perhaps recall the fact that when, more than 26 years ago, 1 first went to England, I was able to show to the late Sir William Preece, then Engineer in Chief of the British Post Office, the transmission and reception of intelligible signals over a distance of if miles by means of short waves and reflectors, whilst, curiously enough, by means of the antenna or elevated wire system, 1 could only get, at that time, signals over a distance of half a mile. The progress made with the long wave or antenna system, was so rapid, so comparatively easy, and so spectacular, that it distracted practically all attention and research from the short waves, and this I think was regrettable, for there are very many problems that can be solved, and numerous most useful results to be obtained by, and only by, the use of the short wave system. Sir William Preece described my early tests at a meeting of the British Association for the Advancement of Science, in September, 1896, and also at a lecture he delivered before the Royal Institution in London on the 4th of June, 1897. On the 3rd of March, 1899, I went into the matter more fully in a paper 1 read before the Institute of Electrical Engineers in London, to which paper 1 would recall your attention as being of some historical interest. DIRECTIONAL TRANSMISSION AT THAT lecture I showed how it was pos, sible, by means of short waves and reflectors, to project the rays in a beam in one direction only, instead of allowing them to spread all around, in such a way that they could not aflFect any receiver which happened to be out of the angle of propagation of the beam. I also described tests carried out in transmitting a beam of reflected waves across country over Salisbury Plain in England, and