114 



BELL SYSTEM TECHNICAL JOURNAL 



Factors Limiting Ultra-High Frequencies 



The limiting factor in the continued extension of the negative grid 

 oscillator to higher and higher frequencies appears to be the dependence 

 of physical size and output power on the operating frequency range. 

 This dependence is well illustrated by the comparison in Fig. 17 of 



Fig. 17 — Some of the tubes, previously discussed, compared in size with a lower 

 frequency tube on the left. 



some of the tubes so far discussed with a standard one-kilowatt tube 

 for which the frequency limit is approximately 75 megacycles. Dimen- 

 sional considerations indicate that the linear dimensions of a series of 

 tubes of optimum design must be decreased in proportion to the 

 operating wave-length. Since the heat dissipating ability depends 

 upon the surface area of the plate, the output (assuming the same 

 efficiency) will decrease as the square of the wave-length. That this 

 is approximately true for the tubes shown in Figs. 5, 7, and 10 may be 

 seen by reference to Fig. 18 where the outputs as a function of fre- 

 quency are plotted on a logarithmic scale. The sloping lines are for 



