Wideband bandpass frequency selective structure based on periodic array of multi-layer strip lines

A three-dimensional bandpass frequency selective structure (FSS), consisting of a 2D periodic array of two vertically placed microstrip lines and one inserted metallic bar, is presented to realize wideband filtering response. Through analyzing the propagation characteristic, it is obtained that two substrate modes and one air mode are excited in each unit cell of the shielded microstrip-line structures. By establishing the equivalent circuited model of the proposed FSS, we observe that the wide passband with four transmission poles is produced by four substrate-mode resonators. Moreover, two air-mode resonators are formed to provide the transmission zeros in upper stopband, thus improving the selectivity of the proposed FSS. A prototype of the proposed FSS is fabricated and tested. The measured 3-dB fractional bandwidth is 60% from 5.3 to 9.8 GHz. Measured results show that this FSS exhibits stable wideband filtering response under a large variation of incident angle.