Vacuum Tubes as High-Frequency Oscillators

HE three types of oscillation generators which at present are the most efficient in the range from 100 megacycles to 3000 megacycles per second will be discussed in the following survey. These are: the negative grid tube which at lower frequencies is the conventional regenerative oscillator, the positive grid or Barkhausen oscillator, and the " m a g n e t r o n " oscillator. The amplification problem will be briefly discussed. Because of the present unsettled state of the theory, only the most elementary and generally accepted part will be included. Much theoretical and experimental work remains to be done before knowledge of the mechanism of oscillation and amplification in this frequency range will be satisfactory. As is often the case, the empirical knowledge of some of these mechanisms has outdistanced the theoretical interpretation. THE NEGATIVE G R I D OSCILLATOR The conventional thermionic vacuum triode, whether it be a large water-cooled power tube or a small receiving tube, may be used as a generator of oscillations varying in frequency from a few cycles per second to some 20 or 30 megacycles with substantially undiminished efficiency and output. In this range the frequency at which a tube is to be employed is a factor of almost negligible importance in the determination of its characteristics and its form. Beyond this range, however, frequency plays an increasingly important part, and as one approaches 300 megacycles, it becomes the most important factor in the determination of tube design.