88 BELL SYSTEM TECHNICAL JOURNAL 



respect to the other component when it reappears at the input hybrid. The 

 two components will consequently combine in the fourth or difference arm of 

 the input hybrid to form a wave equal to the original input wave. 



The circuit of Fig. 3 is only one of a general class of hybrid filter circuits. 

 From one viewpoint these circuits resemble spectroscopes, and, from another, 

 ordinary lumped circuits. We could, for example, replace the band reflection 

 circuits of Fig. 3 with any two identical four-terminal networks. This circuit 

 would still retain its constant resistance character. One of its branches 

 would contain all energy reflected by the four terminal networks; the other 

 would contain all energy passed by them. 



In particular if the two identical filters are of the channel passing type, 

 input waves within this channel will be transmitted by both filters and will 

 combine at the output hybrid and appear in arm C. All of the other channels 

 will be reflected by the filters and thus will appear in arm D of the input 

 hybrid, provided that the assumed 90° phase shift holds over a band which 

 includes all of the channels. 



The particular configuration of Fig. 3 was chosen to minimize the effect of 

 practical limitations. The dropped channel width (20 mc) is only a small 

 fraction of its midband frequency (about 4000 mc) . Consequently when band 

 reflection filters are used in Fig. 3, the change with frequency of the nominally 

 quarter wave sections of guide does not seriously affect the performance of the 

 filter and the impedance match of arm D of the hybrids needs to be good only 

 in the vicinity of the dropped channel. 



The circuit shown in Fig. 3 is a constant resistance network which drops 

 the channel corresponding to the reflection filter. Several in sequence as 

 indicated in Fig. 5 constitute a constant resistance channel branching filter. 

 In the sections to follow we will give an account of the physical configuration 

 and electrical performance of a branching network designed according to this 

 pattern to meet the radio frequency requirements of a typical practical radio 

 relay system containing five 20 mc radio frequency channels, spaced 80 mc 

 center to center. 



TiiE Waveguide Hybrid 



In choosing a hybrid configuration which could be successfully used in the 

 network of Fig. 5, both electrical and mechanical requirements were considered. 

 Since frequency /„ passes through 2n — 1 hybrids it is evident that if accept- 

 able overall performance is to be obtained, the balance and impedance char- 

 acteristics of each hybrid must be excellent. A broad-band balance can be 

 obtained with relative ease by attaching the 'driven' arms (A and B on 

 Fig. 5) symmetrically. Fortunately the strict impedance requirement applies 

 to only one of the two 'driving' arms (C and D), the other being required to 

 transmit only a single channel. 



