Reconfigurable Microstrip Bandpass Filter Based on Spoof Surface Plasmon Polaritons of M-Cusp-H-Type Cells

Here, a compact bandpass filter (BPF) with a reconfigurable rejection based on spoof surface plasmon polaritons (SSPPs) with M-Cusp-H-Type (MCHT) cell is proposed and investigated numerically and experimentally. The dispersion curve of the proposed MCHT cell loaded with capacitors can be split into two modes. Both numerical and experimental results confirm that the transmission characteristic exhibits a harmonics suppression level above 30 dB in the frequency range from 6 to 10 GHz. This enhancement is attributed to the improved propagation properties of SSPPs, which offer advantages such as low crosstalk and tightly confined fields. Furthermore, the designed BPF also features a low cutoff frequency and reconfigurable rejection by incorporating lumped elements, direct current (DC) bias circuits, and varactor diodes. By adjusting the external bias voltage, the designed BPF allows tuning the rejection level, frequency, and bandwidth of the stopband accordingly. An improved and precise forecast of BPF performance is provided through utilization of an equivalent circuit model, along with dispersion relationships of cells and near-field distribution for theoretical analysis. This approach maintains consistent alignment with both simulations and experimental results. The proposed BPF is very compact in size, only about
$0.76~\lambda _{0}\times 0.25~\lambda _{0}$
, where
$\lambda _{0}$
is the wavelength at the center frequency. Thus, the compact, miniaturization and reconfigurable BPF has wide-ranging applications in integrated RF and microwave systems.