750 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



modified to take two non-contacting tuning plungers so dimensioned 

 that they can be moved in very close to the glass wall of the 6BL6. In 

 this way an operating frequency range from 3700-4200 mc was obtained. 



4.2 Qvalitative Verification of Theory 



The results of a qualitative verification of the coupled-resonator 

 reflex klystron theory are given in the form of oscillographic displays in 

 Fig. 16. Photograph (a) shows the (3 + %)-repeller mode of the 6BL6 

 with the secondary cavity completely decoupled. In photograph (b) 

 the coupling iris and secondary Q have been adjusted for a flat topped 

 output curve. The bandwidth of this curve for a variation in power 

 output of ±0.1 db was found to be 58 mc, i.e., considerably greater than 

 the half-power bandwidth of the single resonator case above. Increasing 

 Qsk beyond this point by either enlarging the coupling iris or withdraw- 

 ing the resistance vane from the secondary resonator or a combination 

 of both brings about the condition of critical coupling, illustrated by 

 photograph (c). As explained earlier, this mode-shape results from the 

 admittance plot of the passive circuit forming a cusp in the g-h plane. 

 Comparison between this photograph and the theoretical mode of Fig. 

 9(b) for (Qkf = 1 indicates good qualitative agreement. Coupling the 

 secondary cavity still tighter, i.e., making the value of Qsk greater than 

 unity, causes the formation of a loop in the circuit admittance plot and 

 the consequent load hysteresis shown in photograph (d). Whereas all 

 oscillograms discussed to this point have been obtained with a 60-cycle 

 sinusoidal sweep applied to the repeller, the last one in this group, (e), 

 results from a unidirectional (sawtooth) repeller sweep. The nature of 

 this hysteresis effect has been explained earlier, and oscillograms one 

 might expect to observe with overcoupled resonators for both sinusoidal 

 and unidirectional sweeps were shown in Fig. 10. Again we note good 

 qualitative agreement. 



4.3 Quantitative Verifixxition of Theory 



As further strengthening of the theory underlying the operation of 

 the coupled resonator reflex klystron a quantitative, experimental veri- 

 fication was undertaken. The methods used in connection with this 

 work and the results obtained will form the subject of the sections to 

 follow. To anticipate some of the conclusions which will be presented 

 in the course of this description, let it be said here that the quantitative 

 agreement between theory and experiment was found to be of an order 

 high enough to justify amply the approximations involved in the ex- 



