A strain-insensitive dual-stopband flexible 3D FSS for stable electromagnetic performance under mechanical stretching

A key challenge for flexible frequency selective surfaces (FSSs) is maintaining stable electromagnetic (EM) performance under mechanical deformation. To conform to non-developable curved surfaces, flexible FSSs must simultaneously achieve high deformability and robust EM stability, while the increasing demand for multi-band interference suppression further increases design complexity. Here, we present a dual-band flexible three-dimensional (3D) FSS that integrates a square ring and an S-shaped cross dipole to generate two distinct stopbands. A buckled metallic architecture, combined with a stretchable elastomer substrate, mitigates strain effects and enables conformal integration with curved geometries. Experiments confirm stable dual-stopband filtering over a biaxial tensile strain range of 0% to 12.4%, with resonance frequencies maintained at approximately 3.7 GHz and 5.7 GHz. The underlying strain-insensitive response is elucidated through mechanical simulations, full-wave EM simulations, and equivalent circuit analysis. This work provides a generalizable design strategy for achieving strain-resilient dual-band flexible FSSs, offering practical potential for flexible electronics, wireless sensing, and EM stealth applications.