A COUPLED RESONATOR REFLEX KLYSTRON 717 



pendent of repeller voltage, with frequency linearly related to the latter. 

 Moreover, these conditions should preferably obtain over the widest 

 possible frequency range. A tube possessing such characteristics would 

 prove exceedingly useful in a large number of applications. To list a 

 few: 



(a) Electronically swept signal generator, 



(b) FM deviator, 



(c) Transmitting oscillator in radio relay systems emplo5dng fre- 

 quency modulation, and 



(d) Local oscillator in microwave receivers with wide range AFC 

 applied to the repeller. 



Inability to reaUze this ideal mode shape in a practical tube might 

 make a compromise solution appear acceptable, one consisting of a reflex 

 klystron having a variable mode shape, i.e., a characteristic which 

 could be adjusted to fit a particular need. Thus, appUcation (a) requires 

 constant power output with minor emphasis on frequency-repeller-volt- 

 age linearity, whereas appUcations (b) and (c) demand a high degree 

 of modulation linearity with constancy of power output of no great im- 

 portance. In application (d) the emphasis is on wide electronic tuning 

 mth both variation in power output and non-linearity in the frequency 

 characteristic permissible. 



A method to obtain this variable mode shape was achieved by the use 

 of coupled cavities. Instead of having the bunched electron stream inter- 

 act with the electric field of a single resonator, as is done in the conven- 

 tional reflex klystron, we can present to it the input admittance of two 

 coupled cavities. The resultant mode shape may then be expected to 

 resemble the input impedance of two coupled resonant circuits just as 

 the conventional klystron mode resembles that of a single resonator. 

 Moreover, the input impedance of two coupled cavities can assume a 

 large variety of contours depending on the ratio of Q's of the primary 

 and secondary resonators and on the tightness of coupling between the 

 two. If we were now to construct such a double cavity reflex klystron 

 with provision to vary the secondary Q as well as the coupling coefficient 

 continuously, we would have the means of producing a large variety of 

 mode shapes within a single tube. Depending on the appUcation, the 

 characteristics could then be adjusted to give either a range of flat 

 power output or optimum modulation hnearity or wide electronic tuning. 



As might be expected at this point, a price must be paid for the ad- 

 vantages gained in the coupled cavity approach. It consists of the 

 power expended in supplying the losses due to the secondary resonator. 

 This power subtracts directly from the available useful power and, there- 



