Fully planar high-order frequency selective surface with quasi-elliptical response using multimodal structure

We propose and experimentally demonstrate a fully planar high-order frequency selective surface (FSS) based on multimode resonators (MMRs) with a quasi-elliptic bandpass response characterized by finite-frequency transmission zeros near the passband edges. In our design, these TZs are realized by implementing cross-coupling paths synthesized and guided by a coupling-matrix formulation. The proposed unit cell adopts a resonator-dual-mode-resonator configuration, functioning as a high-order filter low profile, and high selectivity. Coupling-matrix theory is further employed to elucidate the operating mechanism and to analyze the transmission and reflection characteristics, indicating that the filtering performance can be improved by increasing the resonator order and the number of coupling paths. A planar prototype is fabricated and measured, exhibiting four transmission poles and two transmission zeros in good agreement with simulations. The structure achieves a 3-dB fractional bandwidth (FBW) of 11.5% with an overall thickness of only 0.14λ₀, while maintaining low insertion loss and enabling simple, low-cost fabrication.