A bandpass filter (BPF) with a notched band is designed based on the high-order mode of spoof surface plasmon polaritons (SSPPs). The high-efficiency passband is developed from the first high-order mode (Mode 1) of SSPPs. Compared with the fundamental mode having low-pass response, the Mode 1 has the inherent bandpass response; thus, the BPF generated by Mode 1 does not need to add an additional high-pass filtering structure and employ a gradient transition structure for excitation. The Mode 1 of SSPPs can be effectively excited just with a simple trapezoidal transition converting the signals from microstrip line to SSPP waveguide. Moreover, by embedding a U-shaped resonator into the SSPP structure, a BPF with a notched band is constructed, where the location of the notched band will be red-shifted as the length of resonator increases. The measurements of two fabricated prototypes agree with the simulated ones, which verify the feasibility of our design.
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