A Non-reflecting Branching Filter for Microwaves

Abstract

Microwave branching filters are required as integral parts of multi-channel microwave radio relay systems. These filters must have characteristics which are difficult to attain if one attempts to extend familiar lower frequency techniques to the microwave region. A novel network configuration, through which currently anticipated requirements can be met without excessive difficulty, is described in this paper. In this configuration individual constant resistance channel dropping units are formed of appropriate assemblies of two hybrid circuits, two band reflection filters and two quarter wavelengths of line. An assembly of N channel dropping units in cascade then forms an N channel constant resistance branching network. The mechanical and electrical characteristics of a practical five channel branching filter of this type are described. As a result of experience with this prototype filter it can be stated with some safety that these requirements can he fulfilled with a network of this type. Experimentally observed impedance, insertion loss and phase characteristics were fully satisfactory. In addition the circuit appears to be flexible enough both electrically and mechanically to fulfill the various types of systems needs which may be encountered at branch points or when channels must be added or interchanged.