Mode Analyzing Method for Fast Design of Branch Waveguide Coupler

An accurate and efficient method for analyzing the coupling ratio of symmetrical four-port branch waveguide coupler is presented. The method is based on the mode-matching method (MMM) and the odd and even mode analysis, considering the interactions between the fundamental and higher order modes in junctions and discontinuities. It is found that the coupling ratio of the waveguide coupler is depended on the sum of all the branch line height if the center frequency is determined, which is valid for any coupling ratio and any number of shunt branches (≥3) waveguide coupler. Compared to the traditional design method that is complex and only valid under some restrictions, the proposed method has the characteristics of accuracy and universality, and has a simple and intuitive expression. The theoretical work is verified in an experiment by characterizing four fabricated waveguide couplers with a different number of shunt branches.