Low-Profile Water-Based Ultrabroadband 3-D Printed Metamaterial Absorber

A low-profile, dual-polarized water-based metamaterial absorber with ultrabroadband performance is proposed. The design integrates a two-step water block with sinusoidal boundary perturbations and a tapered dielectric shell. The tapered geometry introduces a vertical gradient in effective scattering size, allowing different frequencies to excite different combinations of Mie resonance modes. This mechanism enhances modal density across the spectrum and supports continuous broadband absorption. In addition, a nonuniform gradient impedance transition formed by mismatched tilt angles improves electromagnetic matching. The absorber achieves over 90% absorption from 12.6 GHz to 220 GHz under dual polarization, with robust angular and thermal stability. Measurement results of a 3-D-printed prototype agree well with simulations, validating the proposed geometry-driven multimode absorption mechanism.