Vacuum Tubes as High-Frequency Oscillators * 



By M. J. KELLY and A. L. SAMUEL 



Vacuum tubes as oscillators and amplifiers at frequencies greater than 

 100 megacycles (3 m.) are considered in this paper. The type of con- 

 struction used in a large number of different tubes, and the characteristics 

 of the tubes, are presented. Circuits for operating the tubes also are 

 considered and the theory of operation and the factors limiting ultra-high 

 frequencies are discussed. Principal attention is given to the tubes as 

 oscillators, with brief consideration of the problem of amplification. 



^ I ^HE three types of oscillation generators which at present are the 

 -*- most efficient in the range from 100 megacycles to 3000 mega- 

 cycles per second will be discussed in the following survey. These 

 are: the negative grid tube which at lower frequencies is the con- 

 ventional regenerative oscillator, the positive grid or Barkhausen 

 oscillator, and the ' ' magnetron ' ' 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 amplifi- 

 cation in this frequency range will be satisfactory. As is often the 

 case, the empirical knowledge of some of these mechanisms has out- 

 distanced the theoretical interpretation. 



The Negative Grid 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 de- 

 termination 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. 



When an attempt is made to operate a standard triode at increasingly 

 high frequencies it is found that the output and efficiency begin to 

 decrease. The frequency at which this is first observed will depend 



* Published in November 1934 issue of Electrical Engineering. Presented at 

 Winter Convention of ^, /, E. E., New York City, January 22-25, 1935. 



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