Radio Frequency Amplification

Radio Record, Volume II, Issue 35, 15 March 1929, Page 32

Radio Frequency Amplification

Its Use on Shortwave HE vast majority of short-wave receiving sets now in use consist simply of a rectifier followed by one or two note-magnifying stages. A question that one is often asked is: Cannot high frequency amplification be used in the short-wave set?

The answer is that it can, though nothing like the same degree of magnification is obtainable below 100 metres as upon the medium and long wave-bands. By the use of a specifllydesigned transformer it is possible to obtain a certain amount of magnification at high-frequency from a triode valve with a high amplification factor. The overall magnification, however, is generally so small that it is hardly worth while in view of the extra tuning control that must be introduced. The presence of this naturally complicates searching considerably. ., A-much more promising valve for the purpose is the screen-grid variety of the tetrode. Though it must not be expected that anything like the mag: nification possible on the longer waves will be obtained n the "wavelets," it will give real and very welcome amplification at high-frequency. Hxperiments show that the valve and its accompanying circuits must be very thoroughly shielded by means of & metal sereening-box." Is may he

coupled to the detector valve by either the plain tuned anode or the pardilel feed method. , Further, though an extra tuning condenser is needed, searching is not’ really made more difficult, since the tuning of the high-frequency valve when the screen-grid is used is not very sharp. What one does find is that a transmission whic’ without the H.I. amplifier will be somewhat distorted (owing to the necessity for keeping the set close to the oscillation point) often comes through quite clearly with it, since much less use of reaction is required. In other words, though signal strength may not be much greater, and not the spurious kind due to regenuine H.I. amplification is occurring action. ; a pe y+ HY