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BELL SYSTEM TECHNICAL JOURNAL 



the end of the transmitting period these electrons may migrate from the 

 discharge region, they may recombine with the positive ions, or they may be 

 captured by molecules to form negative ions. Negative ion formation by 

 attachment effectively removes an electron from the discharge because of 

 the great increase in mass. It is an experimental fact that those gases 

 which readily form such ions (of which water vapor is the most common) 

 are the gases which exhibit good recovery in a TR box. This process is 



TIME IN MICROSECONDS 



Fig. 25 — A typical recovery time characteristic for the 721A tube in a TR cavity adjusted 

 to 1.5 db low level loss with a transmitter power level of 100 kw peak 



not deionization in the ordinar}^ sense and it can take place at a surprisingly 

 rapid rate. 



Of course, immediately upon the termination of the transmitting pulse, 

 the cloud of free electrons will cause an extremely high loss to any reflected 

 signal but the loss will rapidly decrease to some limiting value set by the 

 fixed losses in the TR cavity itself. 



A typical recovery curve for the 721A tube is shown in Fig. 25. This 

 curve has a particularly fortunate shape in that the variation in loss with 

 distance, or more correctly with time, is at approximately the same rate as 

 the variation in the reflected signal level with distance for a target of fixed 

 size. The importance of this can be understood by considering the way in 



