Bandpass Half-Mode Substrate Integrated Plasmonic Filters With Steep Roll-Offs

Abstract

This letter presents a bandpass half-mode substrate integrated plasmonic filter (HMSIPF) utilizing spoof surface plasmon polaritons (SSPPs). The HMSIPF consists of a half-mode substrate integrated waveguide (HMSIW) with a periodic square spiral array etching onto the top metal layer. Additionally, two C-ring resonators are etched on both the top and bottom metal layers to introduce an extra transmission zero, enabling a sharp lower roll-off in the filter response without increasing its physical size. A prototype of the HMSIPF was fabricated and evaluated against comparable filters. Measured results demonstrate that the HMSIPF achieves a high transmission coefficient (|S21| > −0.8 dB) and a low reflection coefficient (|S11| < −12 dB) across a wide passband of 5.0-9.0 GHz, corresponding to a fractional bandwidth (FBW) of 57.1%. It also exhibits a steep roll-off with a shape factor of 1.22, along with a compact size of $1.44\lambda _{\mathbf {g}} \times 0.37\lambda _{\mathbf {g}}$ . This HMSIPF design advances the practicality of SSPP-based circuits and holds significant potential for microwave integrated plasmonic circuits and systems, offering excellent filtering performance.