Broadband Miniaturized Bandpass Frequency Selective Surfaces Based on Fractal Transmissive Metasurface Elements

Abstract

In this paper, two kinds of broadband miniaturized bandpass frequency selective surfaces (FSSs) based on fractal transmissive metasurface elements at X band are proposed. FSSs satisfy butterworth response type and stack structures are used to expand bandwidth. FSSs are designed based on the concept of antenna-filter-antenna (AFA) which is employing fractal geometries for patches combined back-to-back and a cross-shaped slot or a double cross-shaped slot etched on the common ground plane with the unit-cell dimension and periodicity of 4.5mm ( $0.15\boldsymbol{\lambda}_{0}$ ). The AFA-based FSSs exhibit superior frequency selectivity and have lower profile than conventional FSSs with an overall thickness of 2mm ( $0.067\boldsymbol{\lambda}_{0}$ ) and 3mm ( $0.1\boldsymbol{\lambda}_{0}$ ) respectively. Prototypes of the proposed FSSs are designed and simulated. To facilitate the design of this structure, a simple equivalent circuit is also presented in this paper. Presented FSSs demonstrate fractional bandwidths of 40% from 8GHz to 12GHz and 64% from 6.8GHz to 13.2GHz, which are corresponding to 3dB bandwidth of 4GHz and 6.4GHz respectively for both reflection (S 11 ) and transmission coefficients (S 21 ) at an incident angle within 60°. Compared to traditional FSSs, the designed FSSs are extremely useful in microwave wireless communication applications with a higher oblique incidence angle across wide bandwidth.