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BELL SYSTEM TECHNICAL JOURNAL 



end of the transmitted pulse and the point where the signal on the radar 

 indicator disappears into the background noise, called the "ring time," 

 therefore measures the over-all performance of the radar. 



An echo box is a particularly useful instrument for radar testing because 

 it measures over-all performance directly, because it permits a rapid tune-up, 

 and because it utilizes the radar transmitter as its only source of power and 

 therefore can be made extremely portable. Figure 12 shows typical ring- 

 time patterns on different types of indicators. In actual practice the ring- 

 time is read in miles on the radar range scale and hence is measured from 



TRANSMITTER 



T-R 

 BOX 



DIR 

 COUPLER 



H ANTENNA 



ECHO 

 BOX 



■PEAK POWER 

 IN ECHO BOX 



RECEIVER 

 NOISE LEVEL 



TRANSMITTER ON-OFF 



h 



RINGING TIME- 



Fig. 11 — Radar test with echo box. 



the beginning of the transmitted pulse rather than the end. The difference 

 is unimportant, however, since standard and limiting values of ringtime are 

 established for operating conditions. It will be noted that the echo box 

 does not return a true echo to the radar, so that the name "echo box" is not 

 entirely appropriate. 



Types and Uses 



Echo boxes are of two general types, tuned and untuned. A tuned echo 

 box is designed to resonate in a single mode adjustable over the operating 



