High-isolation X-Band circularly polarized MIMO antenna with multi-band metasurface for In-out band RCS reduction and vehicular communication

A compact four-element circularly polarized MIMO antenna integrated with a multiband metasurface is proposed, achieving up to 90% absorption across 8.75–15.85 GHz, 22.0–25.5 GHz, and 27.3–27.7 GHz. Both structures are fabricated on an FR-4 substrate (εr = 4.4). The metasurface unit cell measures 6.5 × 7.5 × 3.1 mm3, while the antenna dimension is 29 × 50 × 0.835 mm3 (0.84λ0 × 1.46λ0 × 0.023λ0 at 8.75 GHz). An innovative decoupling approach ensures a 180° phase difference, yielding a wide circular polarization bandwidth of 8.25–12.75 GHz (40%). The antenna exhibits excellent MIMO performance with isolation > 22 dB, ECC < 0.01, DG ≥ 9.99 dB, and TARC < −15 dB. It achieves a radiation efficiency of ≈90%, peak gain enhancement from 6.1 to 7.8 dBi, and RCS reduction of −18.86 dBm2. The measured channel capacity reaches 32.8 bps/Hz at 20 dB SNR, supporting data rates of 112.68 Gbps (16-QAM) and 50.46 Gbps (QPSK). SAR analysis confirms compliance with safety standards. The strong agreement between simulation and measurement validates the design’s suitability for sensing, EMI suppression, imaging, wireless communication, energy harvesting, and satellite-based IoT applications.