A Non-reflecting Branching Filter for Microwaves 



By W. D. LEWIS and L. C. TILLOTSON 



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 tech- 

 niques 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 assembhes 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 branch- 

 ing 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 be 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. 



Introduction 



"Present plans for point-to-point communication by means of micro- 

 -^ wave radio relay systems call for the operation of several radio channels 

 between each pair of repeaters. A proposed frequency plan for the 4030 mc 

 common carrier band (3700 to 4200 mc) specifies channels 20 mc wide spaced 

 40 mc center to center. A possible arrangment for a five-channel radio 

 repeater station is illustrated in Fig. 1. This arrangement is calculated to 

 utilize the available frequency space in an eflftcient and technically sound 

 manner. 



If this channel disposition is to be achieved without a costly increase in the 

 number of antennas and the size of the supporting towers, radio frequency 

 branching networks must be provided which connect the individual trans- 

 mitting or receiving circuits at each repeater point to a common antenna 

 (Fig. 1). If this connection is to be made without excessive loss of power 

 these branching devices must have adequate adjacent channel rejection, low 

 ohmic loss, and good impedance match in the channel bands. An excellent 

 impedance match is especially desirable if circuit disturbances resulting from 

 echoes in the long waveguide Hnes which lead from the filter assemblies to 

 the antennas are to be minimized (Fig. 1). 



Since the type of microwave radio repeater now planned^ obtains most of 

 its gain at intermediate frequencies, the IF amplifiers will reject all spurious 



1 See H. T. Friis, "Microwave Repeater Research", to appear in the April 1948 issue 

 of B. S. T. J. 



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