A Dual-Layer Frequency Selective Surfaces with Tunable Transmission and Fixed Absorption Bands

This paper presents dual-layer frequency selective surfaces (FSSs) with frequency division control function through an integrated tunable transmission window at a lower frequency and an absorption performance at a higher frequency. The bottom frequency selective surface (FSS) layer, configured as a bandpass structure, incorporates a gradient gap square-ring element loaded with varactor diodes. This configuration enables dynamic tuning of the L-band transmission window from 1.26 GHz to 1.9 GHz via varactor capacitance modulation. Simultaneously, the top FSS layer utilizes a square-ring-cross-slot topology. Leveraging the strong reflection characteristic of the bottom FSS at higher frequencies in conjunction with dielectric loss mechanisms, the structure achieves absorption performance within the 5.56 GHz to 5.72 GHz band. Measurement results indicate insertion loss at operational frequencies within the transmission window remains below 1.41 dB, while the absorption peak reaches approximately -30 dB. Close agreement between simulated and measured results validates the proposed design.