Bandpass Frequency Selective Structure With Improved Out-of-Band Rejection Using Stacked Single-Layer Slotlines

In this paper, a class of bandpass frequency selective structures (FSSs) with improved out-of-band rejection is proposed by using stacked slotlines. The proposed FSS consists of a periodic array of slotlines etched in pieces of single-layer printed circuit boards, which are vertically placed along the periodic plane and then stacked together with a certain distance. Due to this special arrangement, slotline resonators are constructed based on the desired propagation modes, thus leading to a stable bandpass filtering response in a wide frequency range. To better understand the operating principle, design procedure of this class of FSSs with uniform slotline unit cells, inductor-loaded uniform slotline unit cells, and stepped-impedance slotline unit cells is presented step by step. In particular, the equivalent transmission line models are established and the even–odd mode analysis method is introduced to intensively analyze the resonant and filtering mechanisms. As two examples, a single-polarized FSS and a dual-polarized one, both based on stepped-impedance slotline unit cells, are designed, fabricated, and measured. Measured results indicate that the proposed FSSs feature miniaturized unit-cell size, wide upper rejection band, and stable frequency filtering response under a large variation of incident angles.