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BELL SYSTEM TECHNICAL JOURNAL 



parable to the values observed for actual tubes. The direct coupling com- 

 ponent of the total flat p(?wer and the gas discharge limited component are 

 found to be additive. Direct coupling power is logically measured in terms 

 of db below the transmitter power level and for the usual TR box is of the 

 order of 60 to 70 db. An abnormal form of direct coupling which may 

 reach dangerous values can occur under certain improper operating condi- 

 tions when the magnetron produces an appreciable amount of power at other 

 than the normal operating frequency. Some of these spurious frequencies 

 may be in the vicinity of the resonant frequencies for these "direct coupling" 



T 



o 



10 20 30 40 bO 



GAP SPACING IN THOUSANDTHS OT AN INCH 



Fig. 24 — The variation in flat leakage power with gap length, measured at 3000 megacycles 



modes in the TR box and so be transmitted without much attenuation. 

 Normally, direct coupling is of interest only in very high power systems. 

 Receiver Self -Protection. The fact was mentioned earlier that a receiver 

 can provide itself with a certain amount of self-protection as a result of its 

 change of impedance with level. This effect is still of use even in systems 

 employing TR boxes. Unfortunately the apparent source impedance at 

 the TR box output terminals is different for the different components of the 

 leakage power so that the self-protection feature cannot be utilized for all 

 components simultaneously. The matter is further complicated by the 

 fact that the converter crystals themselves vary greatly in their impedance 

 and in their variation of impedance with power level. The best designed 

 converters as far as crystal protection is concerned are usually those which 



