Enhanced Mode Conversion and Out-of-Band Rejection in Double-Side Composite SSPPs Structure

In this letter, a novel double-sided composite spoof surface plasmon polaritons (CSSPPs) unit structure is proposed, which consists of a primary grating unit and an auxiliary grating unit. The CSSPPs structure not only realizes mode conversion without transition structure, but also effectively suppresses high-order mode and optimizes out-of-band suppression. The primary grating structure and auxiliary grating structure of the CSSPPs unit introduce an extra phase difference, which makes the spatial distributions of the electric and magnetic fields of the quasi-TEM waves separate from each other, and converted into TM waves, realizing the mode conversion without transition structure. Furthermore, regulating the phase difference between the auxiliary grating and the primary grating can gradually eliminate high-order mode and optimize out-of-band rejection. In order to better verify the scalability and excellence of the structure, a miniaturized bandpass filter by introducing short-circuited branches is designed. The measured results demonstrate that the filter has an insertion loss of less than 1.06 dB in the passband range of 4.38 - 7.33 GHz and a rejection level of more than 35 dB in the range of 8 - 18 GHz.