Design of a Compact Tri-Band NZI Metamaterial Absorber for Stealth, Next-Generation Radar, and Terajet Beamforming Applications

Metamaterials have emerged as powerful platforms for manipulating electromagnetic (EM) waves and enabling highly efficient absorption using relatively simple geometries. Among them, absorbers exhibiting near-zero refractive index (NZI) characteristics are particularly attractive for modern defense and communication systems due to their ability to enhance stealth performance and reduce radar detectability. However, achieving broadband, multi-resonant absorption while maintaining NZI behavior in a compact and fabrication-friendly structure remains challenging. In this work, a compact broadband metamaterial absorber based on two diagonally arranged octagonal frequency selective surface (FSS) unit cells is proposed. The structure employs a copper-silicon-copper stacked configuration with an overall thickness of 1.37 mm. The design achieves near-unity absorption at four resonant frequencies of$8.04 \text{GHz}, 11.99 \text{GHz}, 20.7 \text{GHz}$, and 22.04 GHz, with absorption approaching 100 %. Additionally, tri-band absorption bandwidths of$0.25 \text{GHz}, 1.08 \text{GHz}$, and 2.21 GHz are obtained. The effective permittivity, permeability, and impedance are retrieved from simulated transmission and reflection coefficients, confirming NZI behavior at resonance. Owing to its broadband performance, compact profile, and structural simplicity, the proposed absorber is promising for stealth, advanced antenna, and next-generation communication and sensing applications.