GAS-DISCHARGE TRANSM IT-RECEIVE SWITCH 



69 



mean free path of an electron is in general of the same order as the distance 

 between the electrodes but that very few electrons are able to reach the 

 electrodes because of the very rapid reversals in the r-f field. Electrons 

 therefore oscillate rapidly to and fro, losing energy to the neutral gas mole- 

 cules and to positive ions through occasional collisions The positive ions 

 do not contribute in any substantial way to the discharge current because 

 of their large mass and correspondingly low velocity. The r-f voltage drop 

 across the discharge is maintained at a relatively low value by the formation 

 of more ions and free electrons by collisions between electrons and neutral 



< 



5 80 



< 20 



0.2 



0.4 0.6 O.S 1 2 4 6 8 10 

 WATER VAPOR PRESSURE IN MM OF MERCURY 



l''ig. 23 — The effect on leakage power of the addition of water vapor to 20 mm of hydrogen 



in the 721A type tube 



molecules as soon as this voltage rises above some critical value. Measure- 

 ments indicate that the voltage drop across the r-f gap is of the order of 80 

 to 100 volts for a typical TR tube. The variation in voltage drop with gap 

 length may be inferred from the flat power measurements recorded in Fig. 

 24. 



Direct Coupling. At very high transmitter power levels a third component 

 of leakage power is observed which is directly proportional to the trans- 

 mitter power. This component is usually called "direct coupling". It is 

 due to the transmission of power through the cavity in modes which do not 

 have voltage maxima at the gas discharge gap. It can therefore be ob- 

 serv-ed even when the gap in the tube is short circuited. In fact measure- 

 ments made under such short-circuited gap conditions yield results com- 



