A 3D-Printed Conformal Gradient-Impedance Matching FSS-based Absorber for Wideband RCS Reduction

In this letter, a polarization-insensitive frequency selective surface (FSS)–based gradient impedance-matching absorber is presented for broadband radar cross-section (RCS) reduction. The proposed unit cell comprises a modified circular resistive loop printed on the top surface of a three-dimensional (3D) dielectric substrate, where the top side is planar, and the bottom side is an ink-coated solid hemispherical structure, and the overall structure is backed by a metallic ground plane. By optimizing the hemispherical geometry, a gradual impedance transition is achieved within the lossy layer without increasing the overall profile height, thereby enabling broadband absorption. The proposed absorber is implemented on three different surface configurations: a planar surface, a cylindrical (single-curved) surface, and a circularly paraboloid (double-curved) surface, and the 10-dB RCS reduction bandwidths covering 4.07 to 12.42 GHz (having a fractional bandwidth of 101.27%) are obtained for all three geometries. The key novelty of this work lies in the effective utilization of the inherent air gap within the lossy layer through a gradient impedance-matching strategy to achieve broadband absorption and its subsequent demonstration on a conformal geometry, resulting in a fully functional curved absorber. The concept is further validated through an equivalent circuit model and supported by theoretical and mathematical analysis of the impedance transition mechanism. All three configurations exhibit polarization-insensitive performance and maintain stable RCS reduction for oblique incidences up to 40°. Experimental results from fabricated prototypes show good agreement with full-wave simulations, thereby validating the proposed design methodology.