High-Efficiency, Broadband Converter From A Rectangular Waveguide TE10 Mode to A Circular Waveguide TM01 Mode for Overmoded Device Measurement

To accurately measure the performance of overmoded devices used in high-power microwave systems, a novel structure that effectively converts the TE10 mode in a rectangular waveguide to a high-purity TM01 mode in a circular waveguide is proposed. The circular TM01 mode is excited by a hybrid rectangular E-T waveguide structure, which uses a circular waveguide as the E element. Convex slots loaded at the rectangular waveguide, axially symmetric about the cylindrical waveguide, are introduced to suppress the competing lower order TE11 mode. A tuning stub loaded at the rectangular short wall is used to increase the frequency tuning range. The scattering matrix technique was used to study the mode converter along with the microwave network analysis method using some generalizations of the input port reflection and conversion efficiency, which were verified by the numerical full-wave simulation. Based on the theoretical analysis, an X-band TM01 mode generator is designed and measured. Low-power tests show that, when the length of the tuner increases from 5.7 to 11.7 mm, a wide frequency tuning range of ~500 MHz is obtained with a conversion efficiency of ~99%. Measured results and the theoretical analysis are in good agreement.