622 



BELL SYSTEM TECHNICAL JOURNAL 



mission line which, for example, may be a waveguide having a characteris- 

 tic impedance Zoy, connected through an ideal transformer to an output 

 line having a characteristic impedance Z„x. The output line is connected to 

 the transformer at the point Xo, where Xo represents the gap terminals of the 

 diode. Suppose for the moment that provision has been made for connect- 

 ing the output line at the point in the circuit normally occupied by the 

 cathode-anode planes of the diode. This can be done by means of the 

 special testers shown in Fig. 4. In these testers the anode has been omitted 

 and provision has been made for attaching a coaxial line across the gap 

 between the cathode and anode planes. The diodes used in later tests were 

 identical wdth the device of Fig. 4, except that the coaxial output fitting was 

 replaced by a sheet copper anode. 



Referring again to Fig. 3, assume that the output line is shorted at point 

 .To . If power is introduced in the input line at the left, a standing wave 

 pattern in the input line will pass through a minimum at some point jo . 



r 



--Ax-- 



OUTPUT 



Fig. 3 — Equivalent circuit of diode measuring equipment. 



If the short circuit is now moved to the right by an increment Ax, the stand- 

 ing wave minimum will move by an increment Ay. The relation between 

 A.x; and Ay is given by the following equation: 



1 , 27rAv 



cot 



27rA.T 



<p Bo 



(1) 



where Xy and Xx are the respective wavelengths in the two lines (which may 

 not be equal if, for example, one is a coaxial and the other is a waveguide). 

 <^ is the transformation ratio of the ideal transformer, and ^o is the effective 

 leakage susceptance of the tube and transformer referred to the terminals 



at Xo . If -^^ is plotted as a function of ^^^ on cot-cot coordinate paper, 



\y Ax 



a straight line is obtained whose slope m is 



and whose ordinate intercept p is 



p = —4>BoZqii 

 A typical cot-cot plot is shown in Fig. 5, 



(2) 

 (3) 



