NON-REFLECTING BRANCHING FILTER S5 



simple parallel ox series connections and simple lumped circuit elements do 

 not exist. Although in the interests of flexibility it would be desirable to 

 add or substitute individual channels without affecting other channels in a 

 branching filter, the convenient possibility of doing so at a high impedance 

 level on vacuum tube grids is not yet available in the microwave region. 



A satisfactory two-channel waveguide branching filter has been constructed 

 following partially 'classical' methods. This filter, designed for the New 

 York-Boston experimental radio relay system, is composed of two channel- 

 passing cavity filters each connected to a common input line through one arm 

 of an E plane Y junction, the waveguide analogue of a series connection 

 (Fig. 2). This solution, although relatively simple where only two channels 

 are required, becomes quite complex when more than two are involved, since 

 in every channel the sum of the interactions of all the inactive filters on 



Fig. 2 — Branching filter for New York-Boston experimental radio relay system. 



transmission through the active filter must be zero. It is evidently not easy 

 to satisfy this condition, particularly since in doing so one must take accurate 

 account of the change with frequency of the effective phase shift of all wave- 

 guide connecting Hnes. And even if such a solution were found it would be 

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

 again for every change in channel arrangement. 



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

 it became apparent that a more flexible method of microwave filter con- 

 struction should be found. Constant resistance filters, which provide dis- 

 crimination by absorbing or diverting the unwanted incident waves rather 

 than by reflecting them, can be useful in any frequency range. In the 

 microwave region, where the shortest connecting pip)e may be many wave- 

 lengths long, constant resistance devices are particularly helpful. Accord- 

 ingly a constant resistance channel-dropping network was devised which 



