10-19] 



DUPLEXING SCHEMES 



571 



tion, taking advantage of the directional characteristics of the antennas. 

 Seldom is this a very practical solution in airborne systems. The method 

 requires two separated antennas located where space and weight are at a 

 premium; it poses problems of interconnection, as well as problems of 

 alignment and parallax between the antennas. A preferred solution is to 

 employ some duplexing device or circuitry which permits use of a common 

 antenna for simultaneous transmission and reception. 



There are several linear duplexing schemes which might be used in 

 certain applications; however, in general they involve a two-way trans- 

 mission loss of 6 db. Any of the familiar 3-db hybrid junctions can be 

 employed in a linear balanced duplexer^^ as illustrated schematically in 

 Fig. 10-43. The transmitter is shown connected to port 1 of a hybrid ring 



Dummy 

 Load 



Transmitter 



Ring Hybrid 

 Fig. 10-43 Linear Balanced Duplexer Using Ring Hybrid. 



which divides the power equally between arms 2 and 4. The receiver is 

 connected to arm 3 and will be isolated from the transmitter on arm 1 if 

 arms 2 and 4 are properly terminated. As with any linear duplexer, antenna 

 mismatch will cause direct reflection of transmitter power back to the 

 receiver. This reflected component- can be canceled out by an adjustable 

 reactance in arms 2 or 4, but a very frequency-sensitive duplexer may 

 result. To achieve a high degree of isolation between transmitter and 

 receiver in a duplexer of this type the hybrid must be well balanced and 

 the antenna and dummy load well matched. The 6-db loss arises in this 

 type of duplexer because half the transmitter power is "dumped" in the 

 dummy load and only half the received power reaches the receiver. 



Linear duplexing can also be accomplished using ynode multiplexing 

 techniques. Thus orthogonal linearly polarized modes are employed, for 

 example, in circular or square waveguide, transmitting on one and receiving 

 on the other, and relying on target depolarization of the transmitted signal 

 to permit reception of an echo.®^ A variation of this duplexing technique 

 involves transmitting one sense of circular polarization and receiving the 

 other. 



*''See Paragraph 4-3 for a discussion of the effects of the target upon polarization. 



