GAS-DISCHARGE TRANSM IT-RECEIVE SWITCH 



89 



ciently high level to ionize the TR tube gap, or with a magnetron in the 

 equivalent of a radar microwave head. The high-level test bench used for 

 the 724B is shown in Fig. 36. This bench uses a radar microwave head 

 litted to special plumbing. In either case, the leakage power is measured at 

 a specified R.F. level. For production testing, an actual measurement of 

 recover}' time is not used. Instead a test which measures the quantity of 



Fig. 36 — High Level Test liciiLh lur the 724B Vacuum Tube 



water vapor in the tube in a relative way has been developed. This test 

 involves touching some part of the tube envelope with a piece of carbon 

 dio.xide ice, so that most of the water vapor is frozen out forming a small 

 spot of ice on the inside of the tube. Only the hydrogen remains, and the re- 

 sulting change in either the leakage power or the igniter arc drop is indicative 

 of the quantity of hydrogen and of water vapor in the tube. Careful correla- 

 tion must be made between this simple dry-ice test and absolute recovery- 

 time measurements; experience has shown the dry-ice test to be reliable 

 and in the hands of a skilled operator very informative. 



Low-level loss and tuning are checked at such low level that the gas does 



