A Compact Folded Slotline Bandpass Filter Based on SSPPs With Improved Out-of-Band Rejection

A compact spoof surface plasmon polaritons (SSPPs) bandpass filter (BPF) with improved out-of-band rejection is proposed in this letter. By employing a novel folded slotline cell, the filter achieves a 67% reduction in longitudinal size compared to traditional rectangular slotline cells at the same cutoff frequency, and the filter measures <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.88\lambda $ </tex-math></inline-formula>g <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 0.16\lambda $ </tex-math></inline-formula>g, where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula>g is the guided wavelength at the center frequency, which significantly shortens its electrical length. The lower cutoff frequency is adjusted by the microstrip-slot transition structure, and the higher cutoff frequency is determined by the depth of the SSPPs unit cell groove, which forms a BPF with controllable frequency band. In addition, the performance of the filter with and without unit symmetry is also analyzed, indicating that the filter of making symmetry structure can obtain better performance with the same size. The simulation results demonstrate that the proposed filter has a passband of 1.3-4.1GHz, and superior out-of-band suppression. To validate the accuracy of the simulation results, the filter proposed in this study is fabricated and subjected to empirical measurement, and the experimental outcomes confirm the practical feasibility of the proposed design.