A Novel Concept of TE $_{\text{20}}$ -Mode 90 $^{\circ}$ Waveguide Twist With Mode Filtering Characteristics

We present a novel TE $_{\text{20}}$ -mode 90 $^{\circ}$ waveguide twist concept for millimeter-wave (mm-wave) and terahertz (THz) applications. This waveguide twist has the unique characteristic of mode filtering; that is, only the desired TE $_{\text{20}}$ mode can be rotated and output, the fundamental TE $_{\text{10}}$ mode, and higher order modes will be reflected. In the design, the input/output (I/O) channels are orthogonal oversized waveguides, the TE $_{\text{20}}$ mode is converted into a cross-waveguide (cross-WG) with gradually changed arm lengths. Broad bandwidth (BW) and high efficiency can be easily achieved in a compact size. To verify the proposed concept, two 160–270-GHz prototypes based on rectangular waveguides (RWGs) and groove gap waveguides (GGWs) were designed, fabricated, and measured. The RWG-based prototype was milled in one metal block, while the GGW-based one was fabricated in two metal split blocks. The measured results reveal that the two prototypes achieve average insertion losses of 0.21 and 0.36 dB for the interested TE $_{\text{20}}$ mode, and minimum suppressions of 45 and 35 dB for the unwanted TE $_{\text{10}}$ mode over the desired frequency band. The features of polarization rotation and mode filtering make the proposed TE $_{\text{20}}$ -mode waveguide twist be a candidate for compact, broadband, and efficient waveguide systems.