Generalized Synthesized Technique for the Design of Thickness Customizable High-Order Bandpass Frequency-Selective Surface

This paper presents a design guide for the implementation of thickness customizable high-order (N ≥ 2) bandpass frequency-selective surface (FSS). Admittance inverters are used to synthesize the filter response given desired characteristics such as filter type, center frequency, and bandwidth. The spatial inverter layers are essentially electromagnetic coupling interlayers that can be adjusted to customize the thickness of multilayered FSS without degrading the desired filter performance. A generalized equivalent circuit is used to provide physical insights of the proposed design. This synthesized technique can be adopted to deliver a versatile implementation capability of high-order FSS filters using various dielectric spacers with arbitrary thicknesses. Such a technique enables the realization of spatial filters with variable weight and size, while maintaining the desired filter response. To highlight its thickness controllability, a third-order bandpass FSS operating at the X-band (center frequency at f₀ = 10 GHz) with 10% fractional bandwidth is synthesized in four different thickness configurations (electrical size varying from λ₀/12 to λ₀/6). The simulation results of all the four thickness customized configurations deliver the same filter response. To validate the proposed technique, two prototypes among the four design configurations are fabricated and measured in a free-space environment. Simulated and measured results show good agreement.