10-15] 



MISCELLANEOUS MICROWAVE COMPONENTS 



549 



other ports, yet are isolated from each other. The four-port hybrid junc- 

 tions^^ are most common and include the magic-tee, ring hybrid or ratrace, 

 short-slot hybrid,^^ and t\v& folded-tee hybrids. All of these hybrid junctions 

 have the common property of dividing input to one port equally between 

 two other ports, while the fourth port remains isolated from the first. 

 Because of this characteristic of effecting equal power division, they are 

 classed as 3-db hybrids. As such, they also function as 3-db directional 

 couplers. 



The waveguide magic-tee junction 

 shown in Fig. 10-21 will be used to 

 illustrate the action of a four-port 

 hybrid. A signal entering port 1 of 

 this junction will divide equally and 

 in phase between collinear arms 3 

 and 4, as arm 1 is in shunt with these 

 two arms. A signal entering port 2 

 will also divide equally between 

 arms 3 and 4, but with components 

 which are out oj phase, as arm 2 is in 

 series with arms 3 and 4. Arms 1 

 and 2 are isolated from each other 

 because of orthogonality of their 

 respective modes. 



In a properly designed magic-tee, 

 all ports are matched and arms 3 and 4 are also isolated from each other. 

 Isolation between ports of a hybrid is generally realized, however, only 

 when all other ports are terminated in matched loads. The isolation which 

 one measures between ports of a hybrid junction is, in fact, affected by the 

 terminations attached to the other ports of the junction. In commercial 

 units, isolations range from 20 to 40 db. 



The folded-tee hybrids are functionally similar to the junction just 

 described except that the "collinear" arms (arms 3 and 4 in Fig. 10-21) are 

 folded in the H plane in one type, and in the E plane in a second type to 

 obtain a physical arrangement more convenient for many applications. 

 A commercial //-plane folded tee is shown in Fig. 10-22. 



The balance of a hybrid junction, which is a measure of the equality of 

 power division expressed in db, is of importance in many precision appli- 

 cations as in monopulse antenna feeds and in microwave bridges. The 

 quality of balance oi various commercial hybrid junctions ranges from 

 ±0.25 to rhO.l db for a 10 to 15 per cent bandwidth. 



Fig. 10-21 The Waveguide Magic Tee, 

 a 3-db Hybrid Junction. 



32W. A. Tyrrell, "Hybrid Circuits for Microwaves," Proc. IRE 35, 1294-1306 (1947). 

 33H. J. Riblet, "The Short-Slot Hybrid Junction," Proc. IRE 39, 180-184 (1952). 



