NON-KEFLECriNG BRANCHING FILTER 



89 



Because of the reentrant nature of the circuit of f'ig. 5 it is evidently desir- 

 able, if not essential, to employ a hybrid with a convenient mechanical layout. 

 If, for example, a familiar E-H plane junction type of hybrid were used in 

 combination with waveguide filters built in a straight piece of waveguide, 

 twenty-four E or H plane right angle waveguide bends would be required in 

 the construction of each six-channel branching network. To avoid this extra 

 complication and expense a hybrid configuration with 'driven' output arms 

 parallel to the well matched 'driving' input arm was sought. 



The hybrid configuration settled upon was the one shown in Fig. 6. Here 

 the electrical and geometrical requirements discussed above are met simul- 

 taneously by connecting the driving arm C to the symmetrically located 

 driven arms A and B through a smoothly tapered E plane Y junction. The 

 taper is approximately one half wavelength long in the center of the 3700-4200 



INPUT /---nD 



r ^1 -+ 



+> 





-^>-^ 



■K 



vw 



f^l OUTPUT 



■«> 



Q 



HYBRID JUNCTION 



REFLECTION FILTER 

 FOR FREQUENCY f 



VvV DUMMY LOAD 



_|_ QUARTER -WAVE 

 "^ WAVEGUIDE LINE 



Fig. 5 — N Channel branching filter. 



mc band. The driving arm D is connected to arms A and B through a coaxial 

 line. This line is coupled to waveguide D by means of a conventional probe. 

 The center conductor traverses the Y junction space in such a way that it is 

 normal to the electric vectors of guides A, B, and C, and is effectively coupled 

 to A and B but not to C by means of a probe P fastened through it normally 

 (Fig. 6). 



The Band Reflection Filters 



The ideal reflection filter for the circuit of Figs. 3 and 5 would reflect per- 

 fectly within a certain band and pass perfectly outside of this band. However, 

 the ratio of bandwidth to band spacing in a given branching filter (20 mc to 

 80 mc) is such that a sufficiently good approximation to this ideal can be 

 obtained in theory if each reflection filter employs only three resonances, 

 Fig. 7(a). These could be effectively series resonant circuits placed at quarter 



