68 



BELL SYSTEM TECHNICAL JOURNAL 



and having a low-level loss of 1 db would be 30 milliwatts. The corre- 

 sponding flat leakage power for the 724B tube in a 1.5 db box would be 16 

 milliwatts. Actual measured values are usually somewhat less than these 

 figures. As most crystals will withstand flat powers very much greater than 

 this amount, the flat power is normally of much less importance than the 

 spike in contributing to converter crystal failure. 



Since the flat portion of the leakage power represents quasi-steady-state 

 conditions, it is possible to simulate it for purposes of study by the use 

 of a C.W. oscillator. Fig. 21 contains three experimental curves taken at 



2000 4000 6000 8000 lOOOi 



FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 22 — The pressure for minimum leakage power as a function of frequency 



three different frequencies showing the relationship between the flat leakage 

 power and gas pressure. These curves were all taken with tubes filled 

 with hydrogen only. Fig. 22 shows that the pressure for minimum flat 

 leakage is proportional to the frequency. This simple law probably does 

 not apply at frequencies much less than 1000 mc. 



Water vapor is used in commercial TR tubes to improve the recovery time, 

 as will be discussed later. The variation in flat leakage power with partial 

 water vapor pressure as measured on a 721A type of tube containing both 

 hydrogen and water is shown in Fig. 23. These data were taken in a 

 radar system. 



In this connection, it is of interest to note that the characteristics of the 

 gas discharge in the TR box must of necessity be quite different from those 

 that obtain at lower frequencies. Simple calculations indicate that the 



