VACUUM TUBES AS HIGH-FREQUENCY OSCILLATORS 111 



The logical extension of these principles to increasingly high fre- 

 quencies requires the use of closer and closer inter-electrode spacings. 

 Severe mechanical difficulties are encountered. Curiously enough 

 the limiting factor in the power dissipating ability of the tube turns 

 out to be the grid temperature rather than the temperature of the plate 

 as might be expected. This comes about because of the required close 

 grid-filament spacing, and makes necessary the adoption of some 

 method of cooling the grid. One of the writers has constructed a series 

 of tubes in which the grid is a tungsten helix, each turn of which is 

 attached to a common cooling fin projecting through a slot in the plate. 

 This construction simplifies the mechanical problems involved and 

 provides ample grid cooling. Two of these tubes are shown in Fig. 12. 



Fig. 12 — These tubes represent a further extension in design according to the 

 principles under discussion. The smaller one will oscillate at 1200 megacycles per 

 second. 



The larger one will deliver 10 watts at 670 megacycles with an efficiency 

 of 20 per cent and the smaller one will deliver one watt at 1200 mega- 

 cycles with an efficiency of 10 per cent. These tubes are in no sense 

 commercial, the results representing the limit that has been obtaned 

 by specially constructed tubes under controlled laboratory conditions 

 at voltages and currents above those for which the tube would have a 

 long life. With further advances it is reasonable to expect that outputs 

 of this sort will be commercially realizable and that the frequency 



