Bandpass Filter With Reconfigurable Rejection and Deep-Upper-Wideband Harmonics Suppression Using Spoof Surface Plasmon Polaritons of Hollowed-Bow-Tie Cells

A bandpass filter with a reconfigurable rejection is designed and experimentally validated by exploiting spoof surface plasmon polaritons (SPPs) of hollowed-bow-tie (HBT) cells. As a novel structural discovery, the dispersion curve of the proposed C-loaded HBT cell splits into two modes comparing with the traditional bow-tie cell. With properties of propagating on metal-dielectric interfaces and the exponential increasing propagation constant, SPPs waves show advantages of low crosstalk and tightly confined fields, and experimental results verify that the harmonics suppression level of the transmission characteristic is above 20 dB from 6 to 16 GHz. Moreover, integrated with lumped elements, direct current (dc) bias circuits, and varactor diodes, the designed bandpass filter also has a low cutoff frequency and a reconfigurable rejection. By regulating the bias voltage, the rejection level, frequency, and bandwidth of the stopband are tuned accordingly. The cell dispersion relationships, equivalent circuits, and the near-field distribution are given for theory analyses, and all the full-wave simulations validate well with measurements. The proposed bandpass filter lays a ramming foundation for complex reconfigurable integrated microwave systems, electronically controlled antenna feed networks, and adjustable anti-interference systems.