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 λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub>×0.27 λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> (where λ<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> is the wavelength corresponding to the resonance frequency f<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub>) while offering a maximum relative bandwidth of 60%. It achieves over 40 dB of attenuation in the rejection range of 1.5f<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub>-3f<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and maintains exceptional out-of-band rejection exceeding 40 dB at 3f<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub>. 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.