MICROWAVE REPEATER RESEARCH 



213 



and connections do not exist. Where more than two waveguide channel 

 passing filters are to be connected to a common junction the design becomes 

 complex, since in every channel the sum of the interactions of all the inac- 

 tive filters on transmission through the active fiher must be zero. It is 

 evidently not easy to satisfy this condition, particularly since in doing so 

 one must take account of the change with frequency of the effective length 

 of all waveguide connecting lines. And even if such a solution is found it 

 will be valid for only one set of channels, so that the problem must be solved 

 all over again for every change in channel arrangement. 



INPUT 

 STANDING -WAVE RATIO 



♦TRANSMISSION LOSS AT fo IS LESS THAN 0.05 DECIBELS 



|5 



tnO 



ace 



z 



3940 3950 3960 3970 3980 3990 4000 4200 4210 4220 4230 4240 4250 4260 

 FREQUENCY IN MEGACYCLES PER SECOND 



Fig. IV-3. — Input standing wave ratios of filter of Fig. IV-2. 



As a result of these difficulties and after a few attempts to overcome them 

 it became evident that a more flexible microwave branching filter technique 

 should be found. Accordingly a solution on an iterative basis was 

 developed. A channel dropping circuit was devised which, when inserted 

 in a line, could extract or insert one channel while allowing others to pass 

 through without disturbance. This circuit is of the constant resistance 

 type; in other w^ords it operates by diverting energy selectively but not 

 by reflecting it back to the input. Consequently N such circuits placed in 

 sequence do not interact reflectively; they, thus, form an N channel branch- 

 ing filter which is also of the constant resistance type. 



An individual constant resistance channel dropping circuit is illustrated 

 schematically in Fig. IV-4. It is made up of two hybrid^^ circuits, two 



23 For a general discussion of hybrid circuits see W. A. Tyrrell, "Hybrid Circuits for 

 Microwaves", Proc. I. R. E., vol. 35, No. 11, pp. 1294r-1306, November 1947. 



