Abstract
This study presents a tri-band metasurface absorber (MSA) based on a dual elliptical geometric resonator with an enclosed circular structure. Designed on a low-cost FR-4 substrate, the proposed MMA consists of an elliptical resonator with a strip line shape at the top and a full ground plane at the bottom. To achieve tri-band absorption, a multi-resonance topology incorporating two symmetrical elliptical resonators and a metallic strip is optimized. The unit cell measures 5.8 × 5.8 × 1.5 mm³ (0.54λ₀ × 0.54λ₀ × 0.140λ₀ at the lowest frequency of 28 GHz). A detailed parametric analysis is conducted to examine the impact of the unit cell's geometric variations on the reflection and absorption coefficients. Rotating the metallic strip by 90° in the multi-resonant metasurface structure enabled the transition from single-band to tri-band absorption. Under normal incidence, the proposed MMA exhibits three absorption peaks at 28 GHz, 33 GHz, and 38 GHz, with high absorption rates of 99%, 98%, and 99%, respectively. The MSA is further analyzed under transverse electric (TE) and transverse magnetic (TM) polarized waves, demonstrating polarization-insensitive behavior. The unit cell response is also investigated using an equivalent circuit model and surface current analysis. To validate the design, the unit cell is extended into a metasurface array and fabricated for experimental evaluation. The measured results confirm absorption rates of 97%, 95%, and 99.6% at 28 GHz, 33 GHz, and 38 GHz, aligning well with simulated predictions. The combination of high absorption efficiency, polarization stability, and low-cost fabrication makes the proposed MSA suitable for tri-band 5G/6G millimeter-wave applications.