HIGH Q KESOXAXT CAVITIES 



417 



band is thus strictly limited, if ambiguity is to be avoided. This effect, 

 termed self-interference, becomes an important factor as n increases, since 



/2 



(maximum) 



n + 1 



/i ' n 



This maximum value cannot be realized because it is incompatible with other 

 requirements. 



Fig. 5 — Mode chart illustrating meaning of crossing and interfering modes and of operating 



area. 



WTien an undesired mode crosses the main mode within the rectangle it is 

 called a crossing mode. Except in a region close to the crossing point, it 

 acts only to cause ambiguity as already discussed. In the immediate region 

 of the crossing point, however, the cavity is simultaneously resonant in both 

 modes. Violent interaction effects which may seriously degrade the cavity 

 Q frequently occur at such a crossing. 



Methods of Minimizing Effects of Unwanted Modes 



A major problem in the design of a high Q cavity for radar test purposes, 

 in which the Q and frequency range are set by radar system considerations, 

 is to reduce the effects of all the undesired modes without seriously degrading 

 the main TE 01 ;z mode. Among those to be suppressed is the companion 

 TM \\u mode which is inherently of the same frequency. 



