Broadband Directional Coupler Based on Deformed Circular Waveguide for High-Power Application

A broadband oversized circular waveguide directional coupler for high-power applications is proposed in this paper. The coupler is composed of a group of crossed waveguides, including an oversized quasi-circular main waveguide and a rectangular branch waveguide. Angular deformation is introduced into the main waveguide to realize the compact cross-guide structure, which also contributes to an appropriate coupling degree and high directivity in a broad bandwidth. Moreover, the deformation increases the polarization discrimination ability of the coupler as well, making it feasible in a circularly polarized transmission system. The coupler is designed in the Ku band, of which simulation results indicate a directivity over 23.5 dB in the wide frequency range of 10 GHz to 16 GHz, corresponding to a fractional bandwidth of 46.2%. The impact of parasitic modes on the directional coupler is analyzed to comprehensively survey its performance in oversized waveguide transmission lines. For verification purposes, a prototype of the coupler is fabricated and measured. The experimental results show that a directivity over 22 dB is achieved within the bandwidth, and the coupling degree is around −46.7 dB with fluctuation under 0.9 dB. This paper provides an efficient design and analysis method to develop compact and broadband high-power directional couplers.