Compact and Passband Reconfigurable Filter Based on Gate-Type Spoof Surface Plasmon Polariton Lumped With Varactor Diodes

This paper presents a compact passband reconfigurable filter based on a gate-type spoof surface plasmon polariton (SSPP) loaded with varactor diodes. The gate-type SSPP unit cell exhibits excellent slow-wave performance and strong field-binding capability, with its cut-off frequency controllable through geometric adjustments. The designed filter has capable of real-time conversion from stopband to narrow/wide/ultra-wide passband responses, with a tunable frequency range of 1.41-2.57 GHz. The proposed filter maintains a compact size of 0.63 λg×0.27 λg (where λg is the wavelength corresponding to the resonance frequency f0) while offering a maximum relative bandwidth of 60%. It achieves over 40 dB of attenuation in the rejection range of 1.5f0-3f0 and maintains exceptional out-of-band rejection exceeding 40 dB at 3f0. This compact reconfigurable filter holds significant potential for commercial applications, particularly in resource-constrained, high-tech communication systems, and improving bandwidth utilization in the 5G band.