Broadband Resonance-Free Septum Circular Polarizer Operating Above 200 GHz

In this article, a broadband septum circular polarizer with double inline rectangular-to-semicircular waveguide transformers is proposed, which breaks through the theoretical single-mode bandwidth. Meanwhile, a novel waveguide splitting method is utilized, i.e., the common waveguide and inline waveguide transformers are integrally manufactured. This approach avoids the air gap introduced by the traditional split-block technology, and thus solves the problems of orthogonal-modes leakage and mode resonance caused by the waveguide discontinuity. Therefore, the proposed polarizer possesses the merits of broadband, resonance-free, high consistency of orthogonal modes, and low insertion loss (IL). In the polarizer measurement, a circular to rectangular waveguide converter is designed and fabricated. The measured IL of the polarizer is found to be around 0.28 dB in the frequency range of 198–260 GHz, and the in-band return loss is better than 15 dB. Moreover, the isolation of left-hand and right-hand circular polarization ports exceeds 23 dB within 206–260 GHz. The proposed polarizer has wide potential applications in millimeter-wave and terahertz devices such as broadband antennas and radial power combiners.