Rigorous modal-S-matrix design of a new class of broadband 180-degree branch guide couplers

A class of 180 degrees branch guide couplers is introduced which combines the advantages of the ultrabroadband potential of conventional waveguide E-plane multiple-branch couplers with the low-insertion-loss qualities of E-plane stub-loaded phase shifters. Based on the modal scattering matrix method, the rigorous design takes into account both the finite branch heights and the higher-order mode interaction at all step discontinuities. Computer-optimized five-branch three-stub prototypes, designed for 3-dB+or-0.2-dB coupling, for the waveguide Ku-band (12-18 GHz) and Ka-band (26-40 GHz) achieve typically a 180 degrees +or-1 degrees differential phase shift at the output ports, within about 19% bandwidth, as well as more than 30-dB isolation and return loss. The theory is verified by available measured results.<>