Broadband bandpass filter with double‐layered spoof surface plasmon waveguide as main transmission line

Abstract

In this article, double-layered spoof surface plasmon waveguide (DLSSPW) which consists of double-layered metal gratings is used as a main transmission line for realizing a broadband bandpass filter with a center frequency of 1.5GHz and a relative bandwidth of 80%. Four equally spaced short-circuited double-stripped lines are used as resonators and mounted on the main transmission line. Efficient conversion from the quasi-transverse electromagnetic (TEM) waves in the coplanar waveguide to spoof surface plasmon polaritons (SSPPs) in the DLSSPW is achieved by using gradient double-layered metal gratings and sector-structure impedance matching circuit. This filter has lower insertion loss than using single-layered spoof surface plasmon polaritons, as the DLSSPW has tighter field confinement and lower radiation loss. Moreover, due to the slow-wave effect of DLSSPW, the main-part length of the filter is shorter than microstrip-form counterpart by 30%. The measured S-parameters of the plasmonic bandpass filter agree well with the simulated ones, validating the good features.