Compact, Order Extensible and Wide-Stopband Bandpass Filter Based on SIW Cavity With Rectangular Ring Slot

This brief introduces novel architectures of bandpass filters (BPFs) using a substrate integrated waveguide (SIW) cavity with a rectangular ring slot (RRS) for compact size, extensible order, and broad stopband responses. Two bandpass filters, which demonstrate a second-and a fourth-order Chebyshev response, respectively, are realized by employing identical cavities with RRS, without increasing the physical size of the circuit. A rectangular ring slot is etched along the perimeter and a metallic via is allocated towards the bottom edge of the cavity to realize the resonances. The resonant frequencies of different modes are controlled by introducing one or three slots, which enable the realization of the second and fourth-order BPFs with wide stopbands, respectively. The working principles of the filters are illustrated by applying the eigen-mode analysis, field distribution, and the coupling matrix. Finally, a second-order BPF centered at 0.87 GHz, and a fourth-order BPF centered at 0.92 GHz are synthesized, fabricated, and experimentally validated. As compared to the earlier works, the proposed fourth-order BPF offers competitive performance with a low insertion-loss of 0.7 dB, compact size of <inline-formula> <tex-math notation="LaTeX">$0.036\mathbf {\lambda } ^{2}$ </tex-math></inline-formula>, and a wide passband of 22.1%. Additionally, the second- and the fourth-order BPFs achieve wide upper stopband responses of <inline-formula> <tex-math notation="LaTeX">$3.86{f} _{0}$ </tex-math></inline-formula> at a level greater than 20dB and <inline-formula> <tex-math notation="LaTeX">$3.98{f} _{0}$ </tex-math></inline-formula> at a level greater than 30dB, respectively.