Low Upper Cut-off Frequency Bandpass Filter Based on Spoof Surface Plasmon Polaritons

Conventional spoof surface plasmon polariton (SSPP) filters typically feature high upper cut-off frequencies and large dimensions, rendering them unsuitable for low-frequency applications and compact wireless communication systems. This study introduces a bandpass filter with a low upper cut-off frequency, utilizing an SSPP transmission line (TL) designed for microwave frequencies. The proposed SSPP filter incorporates a series of parallel metal plates positioned within the grooves of the SSPP-TL. An equivalent circuit model for a unit cell of the SSPP-TL is developed to derive the relationship between the cut-off frequency and the circuit parameters, elucidating the cut-off mechanism. To validate this relationship, dispersion curves for critical parameters are computed and analyzed using CST and ADS simulations. A prototype of the SSPP filter, based on the proposed SSPP-TL design, was fabricated and experimentally evaluated. Measurement results confirm that the filter achieves an upper cut-off frequency of 2.75 GHz with superior frequency selectivity, demonstrating its potential for applications in low-frequency circuits and wireless systems.