124 BELL SYSTEM TECHNICAL JOURNAL 



Inglevert "micro-ray link" are of this general type.^* Such tubes 

 have been used to produce oscillations up to 3000 megacycles. 



Because of the fact that the severe limitation on the optimum grid 

 diameter is modified by the presence of a tuned circuit within the tube 

 formed by the helical grid helix, tubes of this type are particularly 

 useful at frequencies above 600 megacycles. The former restriction 

 on grid diameter is replaced by the requirement that the expanded 

 length of the grid spiral be approximately 1.24 times the wave-length 

 at which the maximum output is required, and that there be a correct 

 proportioning of the other dimensions of the tube. The dependence 

 of wave-length on the grid wire length is illustrated by the experi- 

 mental data in Table I covering a frequency range from 460 megacycles 



TABLE I 



Dependence of Wave-Length on Grid Wire Length for B.\rkhausen 

 Oscillator With Helical Grid 



Grid Wire Length Optimum Wave-length 



in Cm. in Cm. Ratio 



16.3 13.5 1.21 



18.6 18.6 1.00 



19.7 14.5 1.36 



20.4 18.0 1.13 



21.4 17.5 1.22 



21.3 25.0 0.85 



22.3 20.2 1.10 



25.2 18.6 1.35 



30.6 25.0 1.22 



32.0 23.6 1.36 



32.0 25.5 L25 



33.4 25.0 1.33 



42.6 29.0 1.47 



42.6 29.5 1.44 



42.6 30.2 1.41 



42.6 30.7 1.38 



53.2 43.5 1.22 



80.0 65.0 1.23 



Average 1 .24 



to 2220 megacycles. Graphic evidence of the independence of shape 

 is shown by the largest and the smallest tube shown in Fig. 24 for which 

 the dimensional ratios are nearly the same. The largest tube delivers 

 several watts at 500 megacycles, while the smallest one delivers only a 

 few tenths of a watt at 2500 megacycles. The efficiency in both 

 cases is about one per cent. These dimensional considerations lead 

 to the conclusion that there exists a maximum output at any given 

 wave-length for a tube of a given design and that this output is pro- 

 portional to the square of the optimum wave-length. From this it 

 appears that the only advantage offered by the spiral grid tube 

 over the other type is the simplification in mechanical design which 



