10-19] DUPLEXING SCHEMES 567 



ensure collimation of transmitting and receiving patterns, but to minimize 

 space and weight requirements of the antenna system, a critical factor 

 particularly in airborne system design. To use a common antenna for both 

 transmitting and receiving poses a severe receiver isolation problem because 

 of the extreme differences in signal level between the strong transmitted 

 signal and the feeble echo return from a distant target. Some form of 

 duple xer^^ is required. 



The prime function of a duplexer is to adequately protect the vulnerable 

 crystal mixers, so generally used in radar receivers, from the paralyzing 

 power of the transmitter, and with a minimum insertion loss to both 

 transmitted and received signals. A radar duplexer may be required to 

 isolate a 100 kw pulse from a mixer crystal which would deteriorate if 

 subjected to 100-mw pulses. This is a power ratio of a million-to-one, 

 indicating the need for more than 60 db isolation between transmitter and 

 receiver for the duration of the transmitted pulse. 



Where transmission and reception take place sequentially, as in pulsed 

 systems, this high degree of isolation is readily obtained using transmitter- 

 actuated gas tube switches of one form or another. Duplexing in FM and 

 CW systems, on the other hand, cannot be accomplished by switching, for 

 transmission and reception are simultaneous and continuous; so other 

 means must be employed. Pulsed systems basically require a nonlinear 

 duplexing system which is amplitude selective, while continuous systems 

 require a static duplexing scheme which provides the require high degree 

 of isolation at all times between receiver and transmitter. In broadband 

 continuous systems; the duplexing problem becomes most acute. 



In pulsed radar systems employing a traveling-wave-tube RF amplifier 

 in the receiver, the duplexing problem is considerably relaxed because of 

 the relatively high input power which the tube can tolerate without 

 damage. There are more important considerations, however, which 

 determine the advisability of using a traveling-wave-tube preamplifier in a 

 particular system. These are discussed in more detail in Chapter 11. 



10-19 DUPLEXING SCHEMES 



With few exceptions pulse radars have always employed gas tube 

 duplexers in one form or another. Controlled by the transmitted RF pulse, 

 the gas tube duplexer depends upon the nonlinear characteristic of the gas 

 to perform the necessary switching action. Under the influence of the 

 transmitted pulse, the gas tubes become intensely ionized and effect a 

 switching action to connect the antenna to the transmitter. Upon deion- 

 ization of the gas tubes, the antenna is switched back to the receiver. 



^Tor a detailed treatment of the subject see L. D. Smullin and C. G. Montgomery, Micro- 

 wave Duplexers, McGraw-Hill Book Co., Inc., New York, 1948. 



