High Q Resonant Cavities for Microwave Testing 



By I. G. WILSON, C. W. SCHRAMM and J. P. KINZER 



Formulas and charts are given which aid the design of right circular cylinder 

 cavity resonators operating in the TE 01 w mode, which yields the highest Q for 

 a given volume. The application of these to the design of an echo box radar test 

 set is shown, and practical considerations arising in the construction of a tun- 

 able cavity are discussed. 



Introduction 



A TUNABLE high Q resonant cavity is a particularly useful tool for 

 -^ ^ determining the over-all performance of a radar quickly and easily^* 

 Further, since it uses the radar transmitter as its only source of power, it can 

 be made quite portable. When a high Q cavity is provided with two cou- 

 plings, one for the radar pickup and the other to an attenuating device, 

 crystal rectifier and meter which serve for tuning the cavity not only can 

 an indication of over-all performance be obtained but other useful informa- 

 tion as well. For example, the transmitter frequency can be measured; 

 calibration of the crystal affords a rough measure of the transmitter power; 

 and an analysis of the spectrum can be made by plotting frequency versus 

 crystal current. This information is of particular importance in radar 

 maintenance. 



The Q required for this purpose is quite high, comparable to that obtained 

 from quartz crystals in the video range. For this reason, such cavities 

 have many additional possibilities for use in microwave testing equipment 

 and microwave systems. For example, they may form component parts of 

 a narrow band filter, or be used as discriminators for an oscillator frequency 

 control. 



Resonant cavities are of two general types — tuned and untuned. A tuned 

 cavity is designed to resonate in a single mode adjustable over the radar 

 frequency range. An untuned cavity is of a size sufficient to support a 

 very large number of modes within the working range. Both are useful, 

 but the tuned variety can give more information about the radar and hence 

 has been more widely used. 



While a tuned cavity may be a cylinder, parallelepiped or sphere (or even 

 other shapes), the first of these has been most thoroughly explored by us. 

 It offers the possibiHty of utilizing the anomalous circular mode, described 

 by Southworth^ in his work on wave guides, which permits the attain- 

 ment of high ()'s in quite a small size. In addition, it is easier to construct 

 a variable length cylinder than a variable sized sphere. 



* Superscripts refer to bibliography. 



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