Power Combiners, Impedance Transformers and Directional Couplers: Part III

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  • Author: Andrei Grebennikov
The branch-line couplers or hybrids were firstly described more than six decades ago, however, the problem of their exact synthesis remained a puzzle for a number of years [53]. Initially, the branch-line hybrid was analyzed as a four-arm symmetrical network based on a superposition of the results obtained in the even and odd modes. By writing the even and odd mode matrices together, the characteristic impedances of the branch lines and coupling into different ports can be obtained. A general synthesis procedure which can be applied to any structure of a multibranch hybrid, based on an invariance of the Richard’s variable S = jtanθ to the transformation of S → 1/S apart from a 180° phase change, had become available a decade later. As a result, with highly precise computer-design techniques available for branchline hybrids, it became possible to generate any coupling value in the useful 0 to 15 dB coupling range. Waveguide designs which have been used in large complex feeds for phase-array radars, are compact, highly predictable in amplitude and phase characteristics, and handle very high power. Coaxial, microstrip or stripline implementations of branch-line hybrids provide simple planar structures of moderate bandwidth capability, up to about 2/3 of an octave.