GAS-DISCHARGE TRANSMIT-RECEIVR SWITCH 



85 



satisfactory gases are not useful because of other characteristics. HCl is 

 an excellent TR gas, but is very corrosive. Freon, a common refrigerant, 

 is excellent but is unstable. In general, no gas which contains a solid ele- 

 mentary constituent is a satisfactory TR gas. The most satisfactory gas 

 found was water vapor. It is cheap, stable, and easy to handle. Water 

 vapor alone is not safe to use at a low temperature, so a small amount of 

 hydrogen is added to ensure adequate protection when the water vapor is 

 frozen out. 



POSITION OF CONE AT 

 ELEVATED TEMPERATURE 

 ED) 



Fig. 33— Cross section of a 721A TR Tube, showing the special shape of the temperature 



compensated discs 



The life of a TR box is in general determined by the gas volume. The 

 radio-frequency discharge consumes no gas except at extremely high power 

 levels; the igniter discharge accounts for the greater part of the loss of the 

 gas initially placed in the tube. Reduction of the water vapor to hydrogen 

 by formation of copper oxide on copper parts of the tube seems to be the 

 principal process which goes on. This change results in no change in total 

 pressure until the water vapor is exhausted; thereafter sputtering becomes 

 more important and accounts for a fairly rapid hydrogen clean-up. The 

 life of a TR tube is determined by the igniter current, which is maintained 

 at a value as small as possible consistent with adequate spike protection, and 

 by the volume of the tube. 



