Miniaturized Ultrawide Stopband Single-Band Filter and Large Frequency Ratio Dual-Band Filter Based on Inhomogeneous Half-Mode T-Septum Waveguide

The inhomogeneous half-mode T-septum waveguide (IHMTSG) cavity is analyzed and applied to the design of miniaturized single- and dual-band band-pass filters (BPFs) in this paper. A defected microstrip structure (DMS) is utilized to achieve ultrawide stopband suppression while preserving transmission characteristics through its band-notched effect. The synthesized, simulated, and measured <italic>S</italic>-parameters of the single- and dual-band BPFs show good agreement. The fabricated single-band BPF operates at 5.8 GHz (<italic>f</italic>₁) with a 3-dB bandwidth of 9.5&#x0025; is designed for n46 frequency range, while the dual-band BPF operates at 4.69&#x002F;25.95 GHz (<italic>f</italic>₁&#x002F;<italic>f</italic>₂) with 3-dB bandwidths of 16.8&#x0025; and 14.73&#x0025; is designed for n79&#x002F;n258 in 5G application, respectively. Excellent upper stopband rejection is achieved, with suppression levels of 20 dB extending up to 9.3<italic>f</italic>₁ in the single-band BPF and 11.3<italic>f</italic>₁ in the dual-band BPF, without spurious modes in the mid-band region. The dual-band BPF achieves a large frequency ratio of 5.53. The proposed single- and dual-band BPFs exhibit competitive performance in terms of miniaturization, ultrawide stopband, and high frequency selectivity with clean mid-band characteristics using the in-house silicon-based MEMS photosensitive composite film fabrication process compared with state-of-the-art filters.