A Compact Wideband Filter Based on Spoof Surface Plasmon Polaritons With a Wide Upper Rejection Band

In this letter, spoof surface plasmon polaritons (SSPPs) are developed in a slotted half-mode substrate integrated waveguide (HMSIW) to realize a bandpass propagation feature instead of a traditional low-pass one. We investigate the dispersion properties to analyze the performance of an SSPP unit cell. Parametric sweepings are further performed to reveal the direct relationships between geometry and cut-off frequencies. We design a compact transition by etching gradient slots on a tapered microstrip-to-HMSIW transition. Then, we finally design an SSPP bandpass filter (BPF) with both wide passband and upper rejection band. The measured 3-dB relative bandwidth is 59.25% ranging from 4.24 to 7.81 GHz. The measured upper rejection band can reach up to 16 GHz with a rejection level better than -31.6 dB. The total size of the filter is only 2.69\lambda _{g} \times 0.42\lambda _{g} . This work can provide a new concept to design novel wideband and ultra-wideband (UWB) microwave filters. Significant potential applications of the strategy of developing SSPPs in HMSIW can be found in microwave versatile planar circuits and devices.