D-Band Low-Loss Moderate- and Broad-Band Bandpass Filters Based on Single-Ridge Waveguide Using High-Order Modes for 6G Applications

Innovative classes of moderate- and broad-band bandpass filters (BPFs) based on a single-ridge waveguide (SRW) operating in D-band are reported. The moderate-band BPF is designed by using a two-level stepped-type SRW, in which a high-order mode is excited and the out-of-band rejection performance is enhanced by introducing metal columns on the SRW top wall. The broad-band BPF follows a similar design concept, with an additional stepped-ridge level that merges adjacent high-order modes to shape a wider transmission band. High-order modes lead to larger dimensions at a specific frequency, thus alleviating D-band fabrication challenges, while the stepped-ridge structure provides more modifiable parameters to better control the filtering characteristics. Both filters share a unified structure, hence demonstrating the scalability and adaptability of the proposed approach through minor geometric adjustments. For practical validation purposes, prototypes of both filters are manufactured using high-precision computer numerical control (CNC) milling technology. Their measured results are in fairly close agreement with the simulated ones. Specifically, the moderate-band BPF exhibits a fractional bandwidth (FBW) of 13.9% at a center frequency of 136.5 GHz, while the broad-band BPF shows an FBW of 39.6% at 139.9 GHz. Furthermore, both filter prototypes feature ultralow minimum in-band insertion-loss levels equal to 0.12 and 0.13 dB, respectively, thus representing a significant improvement over prior-art sub-terahertz (sub-THz) BPFs. Owing to their high operational spectral ranges, low in-band insertion-loss levels, and design flexibility in terms of bandwidth, the engineered D-band BPFs show strong potential for sixth-generation (6G) communication applications.