VACUUM TUBES AS HIGH-FREQUENCY [OSCILLATORS 123 



At lower frequencies the efficiency of operation of a correctly de- 

 signed positive grid tube is substantially the same as that exhibited by 

 this tube. The negative grid oscillator on the other hand, as has 

 been shown, increases both its output and efficiency rapidly with 

 decreasing frequency. The positive grid oscillator is, therefore, at an 

 increasing disadvantage at lower frequencies. With the present 

 state of development, the negative grid oscillator will give larger 

 outputs with higher anode efficiencies at all frequencies less than about 

 300 megacycles. 



For frequencies much higher than 600 megacycles, it is found that 

 the power input requirements for efficient operation of tubes having 

 grids of the straight wire type are in excess of that which can be 

 tolerated in the grid structures. Operation at very much less than 

 optimum input results in considerable decrease in output as indicated 

 in Fig. 25. 



Spiral Grid Barkhausen Tubes 



If the grid of a Barkhausen oscillator is in the form of a simple 

 helix, oscillations at frequencies greater than those predicted by the 

 relationship of equation (1) are readily obtained. When so constructed 

 they are called spiral grid Barkhausen tubes. Some experimental 

 models are shown in Fig. 24. The tubes used in the Lympne to St. 



Fig. 24 — Three optimum positive grid oscillators of the spiral grid type, 

 tube designed for 2500 megacycles, largest for 500 megacycles. 



Smallest 



