Design and Measurement of a Sub-Millimeter-Wave Rectangular TE1,0 to Circular TE0,1 Mode Converter

In this article, a sub-millimeter-wave (MMW) rectangular TE_1,0 to circular TE_0,1 mode converter is designed, fabricated, and tested. The mode conversion is achieved sequentially from rectangular TE_1,0 mode to rectangular TE_2,0 mode and then to circular TE_0,1 mode. To realize the mode conversion from rectangular TE_1,0 to rectangular TE_2,0, a compact serpentine mode converter design is investigated. Theoretical analysis and numerical simulations demonstrate that the proposed serpentine mode converter demonstrates high mode conversion efficiency across a wide bandwidth. The simulated transmission characteristics reveal that the rectangular TE_1,0 to circular TE_0,1 mode conversion achieves a 1-dB conversion bandwidth of 96.9 GHz (356.0–452.9 GHz), with high mode purity (S₂₁ for circular TE_2,1 < -20 dB) and low reflection coefficients (S₁₁ < -23 dB). The fabricated mode converter was realized on oxygen-free copper (OFC) using the nano computer numerical control (nano-CNC) milling technology. The cold-test results of the transmission characteristics have an agreement of ±5% over 1-dB conversion bandwidth with the simulations. Within the bandwidth range of 397.2–446.7 GHz, the mode converter exhibits a transmission loss of approximately 5 dB. Compared to a conventional rectangular waveguide (RW), the transmission loss is significantly lower, highlighting the effectiveness of the mode converter. The compact sub-MMW circular TE_0,1 mode converter reveals its advantages for low-loss terahertz (THz) waveguides, providing substantial potential for integration into high-frequency systems.