Theoretical and Experimental Investigations on a Compact and Broadband TE01Oversized Deformed Waveguide Bend

A compact and broadband circular TE ₀₁ -mode oversized deformed waveguide bend for high-power transmission line is proposed in this article. In order to obtain a circular TE ₀₁ mode with high purity output, a deformed waveguide structure, entitled quad-polar waveguide, which can destroy the degeneration between the TE ₀₁ and TM ₁₁ modes and weak mode conversion inside waveguide bend is introduced. The characterization of quad-polar waveguide and its influence of deformation on the mode spectrum are analyzed. The mode coupling coefficients of quad-polar waveguide are theoretically deduced to investigate the mode conversion inside the waveguide bend. Based on the multimode coupling theory, a broadband oversized waveguide bend with propagation efficiency over 95% from 27.6 to 34.1 GHz is designed. A prototype is fabricated and measured. The average transmission efficiency is over 93% according to the S-parameter measurement. The TE ₀₁ mode with high purity output is demonstrated by the near-field test. The high-power performance is investigated by connecting it with a gyrotron traveling-wave tube (gyro-TWT), and the waveguide bend works stably under a maximum output power of 140 kW. In addition to high power and broadband, the proposed waveguide bend features compact, simple structure, and easy to machine.