Broadband bandpass filter and filtering power divider with enhanced slow-wave effect, compact size, and wide stopband based on butterfly-shaped spoof SPPs

This paper introduces butterfly-shaped spoof surface plasmon polaritons (SSPPs) to design a broadband bandpass filter and filtering power divider (FPD) with enhanced slow-wave effect, compact size, and wide stopband. The dispersion and transmission characteristics of the proposed butterfly-shaped SSPP unit-cell are analyzed. It has higher slow-wave features than the conventional rectangle-shaped SSPP unit-cell with same lateral size. Accordingly, compact devices with enlarged confinement of surface waves can be implemented. The proposed filtering structures are realized by etching periodic butterfly grooves on the bottom layer of the substrate and using microstrip-to-slotline transition based on radial stubs. The lower and upper cutoff frequencies of the passband can be adjusted independently by tuning the dimensions of the radial stubs and butterfly grooves, respectively. The designed SSPP bandpass filter and FPD are fabricated. The measured results show a fractional bandwidth of 135.8% and 109.7% with an out-of-band rejection of greater than 31 dB from 6.9 to 18 GHz and 33 dB from 6.6 to 18 GHz for the bandpass filter and FPD, respectively. Moreover, the butterfly-shaped SSPPs achieve a lateral size reduction of about 39% compared to the rectangle-shaped SSPPs. Considering the above advantages, the presented structures provide a highly efficient solution for future telecommunication systems.