74 BELL SYSTEM TECHNICAL JOURNAL 



the transmitter. This same target is represented on the plot as line A' 

 for a transmitter output power of 155 kw, that is being displaced vertically 

 by approximately 12 db to take account of the difference in transmitter 

 power. At this higher power level the target would be visible at a much 

 greater distance (corresponding to 7 microseconds elapsed time) and would 

 remain in view until the target distance corresponded to 2.1 microseconds 

 time. In this case an increase in power by 12 db resulted in an increase 

 in range by a factor of 2.8. While this would indicate that an increase in 

 range can be obtained by increasing the transmitter power, it should not be 

 inferred that an increase in the near-range sensitivity will always result from 

 an increase in power. At any specified range there appears to be a unique 

 value of transmitter power output beyond which the loss in TR box recovery 

 more than offsets any increase in range due to higher output powers. While 

 accurate figures are not available for the 721 A tube, there is some evidence 

 that an output power of 100 kw is already too large for ranges corresponding 

 to elapsed times of 10 microseconds or less. Under these conditions im- 

 proved operation results from a decrease in the transmitter power level. 

 Such an effect has never been observed by the writers with the 724B tube, 

 probably because the transmitter powers available in its operating fre- 

 quency range have usually been somewhat less than that available with the 

 721A tube. 



It should be noted, at this point, that the recovery time does not depend 

 upon the transmitter power only, but rather upon the gas discharge power 

 which is a function of both the transmitter power and the low-level loss 

 adjustment of the TR box as shown by Fig. 16. A very great improvement 

 in near range sensitivity can usually be obtained by increasing the trans- 

 mitter power level and at the same time increasing the low-level loss adjust- 

 ment of the cavity to limit the gas power to a value for which the recovery 

 time is satisfactor}^ This of course increases the ultimate low-level loss and 

 so adversely affects the long-range sensitivity. 



The dependence of the recovery time on the ambient temperature for the 

 721A tube is shown in Fig. 27. The 724B tube is much less temperature 

 dependent. This variation in recovery time with temperature is caused 

 by the reduction in water vapor pressure through condensation, as shown 

 by the identity of tlie recovery curve for a standard 721A tube at — 186° C. 

 with a special tube filled with hydrogen only. 



With continued life the water vapor content of the tube decreases with 

 the corresponding change in the recovery time characteristic. Fig. 28 

 shows the effect with the 721A tube. The dependence of the recovery time 

 on the water vapor content in the 724B tube is shown in Fig. 29. Comparing 

 this curve with Fig. 27, it will be observed that the loss of water vapor has 

 much less effect on the recovery characteristics of the 724B tube than on the 



