The Gas-Discharge Transmit-Receive Switch 



By A. L. SAMUEL, J. W. CLARK and W. W. MUMFORD 



' I '^HE gas-discharge transmit-receive switch has become an accepted 

 -•■ part of every modern radar set. Indeed, without such a device, an 

 eflficient single-antenna micro-wave radar would be nearly impossible. 

 Many of the early radar sets made in this country employed separate 

 antennae for the transmitter and receiver. The advantages of single 

 antenna operation are so apparent as hardly to require discussion. The 

 saving in space or, if the same space is to be occupied, the increase in gain 

 and directivity of a large single antenna is, of course, apparent. But 

 even more important, perhaps, is the tremendous simplification in tracking 

 offered by a single antenna, particularly where a very' rapid complex scanning 

 motion is desired. 



The fact that the receiver needs to be operative only during periods be- 

 tween the transmitting pulses makes single antenna operation possible if 

 four conditions are satisfied. These are: (1) the receiver must not absorb 

 too large a fraction of the transmitter power during the transmitting period, 

 (2) the receiver must not be permanently damaged by that portion of the 

 transmitter power which it does absorb, (3) the receiver must recover its 

 sensitivity after any possible overload during the transmitting pulse in a 

 time interval shorter than the interv'al required by the reflected pulse to 

 arrive back to the receiver from the nearest target, and (4) the transmitter 

 must not absorb too large a fraction of the received power. At frequencies 

 of the order of 700 megacycles and at low power levels these conditions 

 are not impossible of attainment without recourse to any special switching 

 mechanism other than that provided automatically by the usual circuit com- 

 ponents. Conditions (1) and (2) can be met by designing the receiver in 

 such a way that the change in input impedance as a result of overload will 

 cause most of the available input power to the receiver to be reflected. 

 Condition (3) requires careful attention to the time constants of all those 

 receiver circuits which are subject to overload. Condition (4) fortu- 

 nately is automatically satisfied by most transmitters, again as a result of the 

 large mismatch reflections which occur at the connections to the trans- 

 mitter's "tank" circuit when the transmitter is not operating. The United 

 States Navy Mark 1 radar was operated on this basis. 



The speed with which the transmit-receive switch must operate rules out 

 all consideration of mechanical devices, at least for all but the longest range 



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