This paper presents a novel tri-band bandpass filter based on multi-mode spoof surface plasmon polaritons (SSPPs) in microwave frequencies. The combination of multi-mode resonance and cutoff characteristic of SSPPs contributes to a tri-band bandpass response. The proposed SSPPs-based filter consists of two coplanar waveguides, two conversion parts and a SSPPs transmission structure made of five unit-cells. Each unit cell is a multi-mode resonator, which is constructed by a pair of oppositely oriented double-side folded strips, and the ends of folded strips are placed closely to form coupled-stubs. Two distinct passbands divided by a sharp stopband are produced under the cutoff frequency of SSPPs transmission structure. The third passband above the cutoff frequency is introduced by the resonant tunneling effect due to the self-resonant mode of proposed SSPPs unit cell. Dispersion relations, electric field distributions and proposed equivalent transmission line model give a physical insight into the filter operating mechanism. As a demonstration, a prototype of the proposed filter has been fabricated and measured. The measurement results demonstrate that the proposed filter has a tri-band bandpass performance with excellent frequency selectivity, which may be very useful in many applications, such as multi-band wireless systems.
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