NATURE OF POWER SATURATION IN TRAVELING WAVE TUBES 869 



instance), the results are believed to be significant for tubes having the 

 indicated values of 7ro and QC. 



Il HELIX IMPEDANCE 



It is important to the measurements to have an accurate evaluation 

 [of the helix impedance. Several methods of measurement have been 

 discussed in the literature.^^ ' ^^ That described by R. Kompfner was 

 I selected, wherein the circuit impedance is correlated with the beam 

 current and voltage which gives a null in the output signal. When the 

 beam voltage and current are adjusted to give zero transmission for a 

 lossless section of helix (neglecting space charge) CN = 0.314 and 

 5F/Fo = 1/A^. Using the measured length of the helix, and measuring 

 the voltage and current giving the null in signal transmission, we can 

 compute C, and thus the impedance and velocity (synchronous voltage) 

 of the hehx. 



The impedance was calculated by P. K. Tien,^' and the results are 

 compared in Fig. 17. The measured impedance at the high frequency 

 end was much too low until space charge in the beam was accounted for 

 in interpreting the measurements. Fortunately, in the absence of attenu- 



1000 

 800 



600 

 400 



^ 200 

 UJ 



u7 100 

 o 



z 

 < 



Q 

 Hi 

 Q. 



5 



80 



60 



40 



20 



10 



40 80 

 FREQUENCY 



120 160 200 

 MEGACYCLES PER SECOND 



240 



Fig. 17 — Helix impedance as a function of frequency. The impedance was 

 calculated taking into account dielectric loading and wire size. It was measured 

 using the Kompfner dip method, taking account of space charge. 



