718 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



fore, though resulting in broadband operation, greatly improved modula- 

 tion linearity and a number of other useful properties, leads to a definite 

 reduction in output level. Whether this can be tolerated will again de- 

 pend on the appUcation. In many practical cases the performance flexi- 

 bihty inherent in the coupled resonator reflex klystron will more than 

 outweigh this advantage. 



It is the purpose of this paper to present the theory underlying the 

 operation of the coupled resonator reflex klystron, as well as its experi- 

 mental verification. For the sake of completeness, but also in order to 

 emphasize methods of analysis to be used in later sections and not 

 readily found in the hterature, a review of reflex klystron theory will 

 precede the exposition of the coupled resonator problem. In both the 

 single and coupled resonator case, performance analysis will be based 

 on a separate and independent study of the electronic and passive circuit 

 admittance developed across the interaction gap and upon the graphical 

 combination of the two in the complex admittance plane. As a by-product 

 of this investigation, a number of driving point properties of two coupled 

 resonant circuits will be developed which may be found of general net- 

 works interest. Following the theory of the coupled resonator reflex 

 klystron, an experimental tube of this type will be described and a 

 qualitative as well as quantitative verification of the theory given. 

 Oscillograms will be presented showing the advantages of this device 

 when used as a sweep generator, both in the microwave band and at 

 lower frequencies. Additional applications will be indicated in the hope 

 that others may try them. 



2.0 SMALL SIGNAL REFLEX KLYSTRON THEORY 



This section will be devoted to a brief review of the small signal 

 reflex klystron theory. Emphasis wifl be on concepts leading to the 

 equivalent circuit representation and to the graphical admittance-plane 

 analysis. Both of these and particularly the graphical approach will 

 later be used in the investigation of coupled cavity behavior. 



As stated before, the operation of the reflex klystron is the result of 

 the interaction between a bunched electron stream and the varying 

 electric field existing inside a resonant cavity. In circuit language, this 

 amounts to an interaction between an active and a passive element. 

 The active one due to the electron stream is termed electronic admit- 

 tance, and the passive one is the input admittance of the resonator. 

 Derivations for the expression describing the electronic admittance may 

 be readily found in the literature.^ It will not be repeated here. The re- 



