///(;// {> RKSOX.WT CAVITIES 425 



greater it is generally not possible to locate such areas. An operating area 

 for 9 kmc is shown on Fig. 7. Nine crossing modes and twelve interfering 

 modes exist. For 25 kmc the crossing modes run into the forties with 

 hundreds of interfering modes. 



Suppression of the undesired modes requires a thorough knowledge of 

 their field configurations and a number of effective techniques which may be 

 applied on a practical engineering basis. Several examples are cited in later 

 sections. 



Since decrement is an important characteristic of these cavities, espe- 

 cially when applied to radar test sets, the uniformity of the decrement over 

 the frequency range or "flatness of response" may be a significant design 

 requirement. It will be seen from Fig. 3, that the MS factor of the wanted 

 mode is not constant with varying D/L. In fact, if it were, the decrement 

 would increase as the 3/2 power of frequency. 



There are at least three attacks on this "flatness" problem: (1) to operate 

 on the sloped portion of the MS curve in such a manner that its characteristic 

 will tend to be complementary to the change with frequency; (2) to obtain 

 compensation by varying the coupling with frequency — generally accom- 

 plished by selecting an appropriate coupling point along the side wall of 

 the cavity; and (3) to overplate a portion of the cylinder's interior surface 

 with a material of higher resistivity such as cadmium. For this third 



s 



method, the formulas for Q^ of Table I are no longer applicable since they 



A 



assume a uniform resistivity of the cavity walls. 



Thus, it will be seen that the final design of a cavity resonator is a com- 

 promise between a number of desired characteristics: 



a) A cavity of minimum volume for a given Q. 



b) A cavity having a minimum of extraneous responses of types dithcult 



to suppress. 



c) A cavity with compensation for flatness of decrement. 

 Engineering judgment is required to weigh the emphasis on each of these 

 requirements which at times may be mutually exclusive. 



Some Pr.a.ctical Considerations 



Physically realizing the theoretical ciiaracteristics just described to obtain 

 a satisfactory cavity brings forth a host of practical design problems. A 

 number of these will be discussed in this section. 



Description of Echo Box Test Sets 



The schematics (Figs. 8 and 9) and photographs (Figs. 12 to 14) show the 

 components and various construction methods of echo boxes in the 3 and 

 9 yfemc bands. The cavity itself may be spun, drawn or turned of material 



