212 BELL SYSTEM TECHNICAL JOURNAL 



Because of IF amplifier band-pass characteristics, suppression requirements 

 on the filter, except possibly in the vicinity of receiver image bands, are not 

 large. 



Microwave band-pass filters consisting of one or more cavities arranged 

 in sequence along a waveguide have been known for some time. The fre- 

 quency, bandwidth and discrimination characteristics of such filters can 

 all be chosen within wide limits by appropriate design of the cavities and 

 the means for coupling to them. These filters are analogous to lumped- 

 circuit channel-passing filters and can in principle, like them, be connected 

 in groups to provide a branching network. 



i20° E- PLANE 

 WAVEGUiOE JUNCTION 



CHANNEL B 

 OUTPUT 



Fig. IV-2. — Photograph of a branching filter for an experimental radio relay system. 



Several successful two-branch networks have been designed and con- 

 structed in this manner. One of these, developed for the New York- 

 Boston circuit*, is illustrated in Fig. IV-2. Here two three-cavity filters 

 are connected to an E plane Y junction, the waveguide analogue of a series 

 connection. The two filters are tuned to different bands and each is con- 

 nected to the junction through a line of length such that it causes no disturb- 

 ance in the channel of the other. The electrical characteristics of this 

 filter are plotted in Fig. IV-3. 



Problems connected with the design of suitable microwave branching 

 filters with more than two branches evidently differ considerably from pre- 

 vious filter problems. Channel passing networks which can be connected 

 in series or parallel to form a channel branching filter can be designed at 

 lower frequencies on the basis of lumped circuit theory and built of coils, 

 condensers and resistances, but in the microwave region simple elements 



* Developed by the group concerned with high-frequency filter design headed by 

 A. R. D' heedene. A large part of the research underlying the design of these filters was 

 performed by W. W. Mumford. Prior to the war a considerable amount of research on 

 the band-pass type of waveguide filter had already been done by A. G. Fox. 



