10-20] 



SPECIAL PROBLEMS OF COHERENT SYSTEMS 



573 



mitter and receiver using a linear duplexer is dependent on the quality of 

 antenna match. With a perfect circulator, the maximum transmitter- 

 receiver isolation achievable is determined by the mismatch on the antenna 

 arm. The degree of isolation which could be obtained using a perfect 

 circulator as a duplexer is shown in Fig. 10-44 as a function of the reflection 

 coefficient of the antenna arm. It is plain that to achieve high isolation 

 the antenna arm must be exceptionally well matched. For the same 

 antenna arm reflection coefficient an additional 6-db isolation between 

 transmitter and receiver is obtained by using the linear balanced duplexer, 

 because of the split of both the transmitter power and the reflected power. 

 But of course system performance is down by 6 db in this case. 



The circulator is also useful for duplexing in pulsed radar systems where 

 it is possible to use a supplementary TR tube to obtain much greater 

 receiver isolation than is provided by the circulator alone. ^^ The advantage 

 of such a duplexer arrangement is that the circulator greatly reduces the 

 RF power incident upon the TR tube. The result is considerably increased 

 tube life. Reduced ionized intensity of the TR tube effects a reduction of 

 recovery time, thus permitting close-range reception of weak signals. 



As shown in Fig. 10-45 the TR 

 tube in the circulator duplexer might 

 be replaced by an electronically con- 

 trolled ferrite device to provide at- 

 tenutating or switching action which 

 can be synchronized with the trans- 

 mitted pulse. The ferrite element 

 can also function as an RF gain 

 control or as a sensitivity-time con- 

 trol (STC) which regulates receiver 

 serrs-itTVTty as a function of target 

 range. 



Certain other advantages are 

 gained in using a circulator duplexer. 

 The circulator provides load isola- 

 tion for the transmitter, as antenna 



reflections will be absorbed in the dummy load (Fig. 10-45) when using a TR 

 tube, or possibly in the ferrite control element when one is used. Local 

 oscillator radiation is also reduced, as it too is absorbed in the dummy load. 



I Antenna 



Fig. 10-45 Circulator Duplexer with 



Auxiliary TR Tube or Ferrite Device 



to Improve Receiver Isolation. 



10-20 SPECIAL PROBLEMS OF COHERENT SYSTEMS 



By a coherent system is meant one in which a phase or frequency 

 comparison is made between the transmitted signal and the received echo. 



*8"X-Band Ferrite Duplexer," Electronic Design, August 1, 1956, pp. 32-33. 



