Design and Test a Broadband Circular Waveguide Directional Coupler

Based on the theory of small aperture coupling and the principle of phase superposition, a broadband circular waveguide directional coupler is designed to measure the output power for Gyro-TWT at Ka-band. Unlike conventional coupler, the coupler's secondary waveguide adopted the nonstandard rectangular waveguide instead of standard waveguide to expand the bandwidth, improve the directivity and reduce the coupling fluctuation in the whole Ka-band. Through theoretical analysis, a 32 apertures circular waveguide directional coupler is designed by the high frequency simulation software HFSS and tested by vector network analyzer, the simulated and tested result is in good agreement, the coupling degree 40 dB, the fluctuation of the coupling less than 1.5dB and the directivity over 30dB is obtained at the whole Ka-band. The result demonstrates the superior performance of the coupler.