///(;// {) KKSOXAM CAVITIES 433 



merging point of the cavity signal and the noise will show proportional 

 changes in ringtime. Ifencc, the ringtime indication measures these two 

 factors on which the radar's ability to discern real targets so largely depends. 

 The exponential buildup and decay of the charge in the cavity occur at a 

 rate determined by the working decrement of the cavity. As mentioned 

 previously, a decrement of about 3 db per microsecond is a satisfactory value 

 for the 3 kmc and 9 kmc bantls. A one microsecond change in ringtime 

 (roughly one-tenth mile) would, therefore, represent a change in system 

 performance of 3 db. 



I'nijormiiy Control and Expected Ringtime 



By introducing an adjustment for the working Q of the cavities it is now 

 possible to control the uniformity of the manufactured product to very 

 close limits. Other improvements have also been incorporated which insure 

 that boxes which have been made alike as to Q will similarly give uniform 

 ringtime indications on a test radar. If the test sets are all alike as to ring- 

 time, it is then possible to quote an "expected ringtime" for each of the 

 various radars to be serviced by the echo box. Initially a measuring tool 

 indicating relative changes in day to day operation of the radars, the uni- 

 formity provision with its "expected ringtime" has made the echo box test 

 set an absolute measuring instrument of moderate precision. 



Other Uses 



In addition to its use as a measure of over-all performance of a radar, a 

 significant number of diagnostic tests may be performed when trouble de- 

 velops, which aid in rapidly locating the source. One such test is spectrum 

 analysis. The extreme selectivity of the high Q cavity permits examination 

 of the spectrum of the pulsed wave and from this may be deduced charac- 

 teristics of the pulse, including pulse length. Multiple-moding of the 

 magnetron circuit is easily shown by this analysis. 



The meter of the test set gives a relative indication of the output power of 

 the radar and this in itself assists greatly in segregating transmitter troubles 

 from receiver troubles. 



Also of importance is the use of the echo box as a frequency meter. The 

 high Q of the cavity plus the fine control of the drive mechanism and the 

 direct reading dial give excellent results (comparable to that of a wave- 

 meter). 



Acknowledgement 



The design of resonant cavities is a difiicult and complex art. In bringing 

 it to the present state, the number of individuals who have made significant 

 mathematical, theoretical, engineering and mechanical contributions is so 



