Compact Muti-Frequency Band-Pass Filters Based on High-Order Mode of Folded Line Spoof Surface Plasmon Polaritons Waveguide

We proposed a folded line spoof surface plasmonic polaritons (SSPPs) waveguide that allows for efficient and strongly constrained microwave SSPPs transmission. The dispersion characteristics of SSPPs waveguides based on folded line unit is investigated. Compared to conventional rectangular-grooved SSPPs waveguide with the same cut-off frequency, the lateral dimension of the folded line SSPPs waveguide can be reduced to 56%. The results show that the folded line SSPPs waveguide has stronger field binding as well as better miniaturization advantages. In order to validate the proposed design, a dual-band band-pass filter is designed based on the high-order mode of the proposed unit and we then add an asymmetric structure to achieve a tri-band band-pass filter with good performance. In addition, a band-pass filter with wide stopband between two pass bands is designed by incorporating an interdigital finger in the folded line unit, whose width of the stopband can be increased by 50% compared to the filter without interdigital finger. By optimizing the length of interdigital finger, two notch bands are respectively obtained in two pass bands of a dual-band pass-band filter. At the same time, the stopband is also widened. Four proposed filters are fabricated and the measurement results are in good agreement with the simulated ones, verifying the feasibility of the structure. The proposed SSPPs structure can significantly enhance the flexibility and universality of the filter. Additionally, these filters offer compact structures, wide stopbands, and hold immense potential for application in integrated devices.