568 ANTENNAS AND RF COMPONENTS 



Ionization time is almost negligible, while deionization or recovery time may- 

 be several microseconds or more. The recovery characteristic of a duplexer 

 plays an important role in the short-range performance of a radar. Too long 

 a recovery time will prevent detection of small targets at short range, while 

 too short a recovery time may result in receiver overload on short-range 

 targets. 



The branched duplexer and the balanced duplexer are the most widely used 

 gas tube duplexers. The branched duplexer employs a TR and ATR tube to 

 effect switching. On being ionized, the TR tube provides the high atten- 

 uation between transmitter and receiver required to protect the mixer 

 crystal. On deionization, it is the function of the ATR tube to direct the 

 received signals through the TR tube to the receiver and at the same time 

 prevent them from entering the magnetron branch. Very high-power 

 systems sometimes employ a third tube called a pre-TR tube which func- 

 tions as a second TR tube in cascade with the first to provide the greater 

 isolation needed with the higher power. The branched duplexer has certain 

 disadvantages — notably, rather limited bandwidth. In its un-ionized 

 state the ATR tube presents a serious mismatch to the magnetron, and 

 under certain phasing conditions, troublesome "moding" can occur in the 

 magnetron. ^^ For this reason the magnetron-to-ATR spacing becomes an 

 important factor in the design of a branched duplexer. 



A number of advantages over the simple branched duplexer are offered 

 by the nonlinear balanced duplexer. Now being used extensively, the 

 nonlinear balanced duplexer provides broadband operation, eliminates the 

 ATR tube and its mismatch problem, provides somewhat better crystal 

 protection, and generally introduces somewhat less loss than the branched 

 duplexer. The balanced duplexer employs two TR tubes suitably mounted 

 between two 3-db hybrid junctions in a bridge arrangement. Balanced 

 duplexers have been made using many types of hybrid junctions, but the 

 slot hybrid duplexer^^ has found widest acceptance because of its compact- 

 ness and unexcelled broadband performance, h dual TR tube having a 

 common gas fill is now commonly used in the slot hybrid duplexer to 

 obtain best bandwidth through closely balanced characteristics. Duplexers 

 which incorporate solenoid-operated shutters in the TR tubes are available 

 to provide crystal protection against stray radiation whenever the radar 

 is turned off. 



An integral slot-hybrid balanced duplexer for X-band, incorporating a 

 dual TR tube with shutters, is shown in Fig. 10-40. The signal path in such 

 a slot hybrid duplexer for the transmit condition is shown in Fig. 10-41a. 

 On entering the duplexer, the transmitter power is split by the 3-db hybrid 

 and, on ionizing the TR tubes, is reflected. However, because of the 



^Magnetrons and Traveling-W a'oe Tubes, Tube Division, Radio Corporation of America, 

 Harrison, N. J., 1956. 



