Compact Four-Port Metasurface for Tri-Band Operation in X-Band MIMO Applications

This paper presents the design, fabrication, and experimental validation of a compact four-port metasurface for tri-band X-band multiple-input multiple-output (MIMO) applications operating at 8.75 GHz, 9.75 GHz, and 10.5 GHz. The proposed structure employs a scalable unit-cell configuration to enhance radiation performance while maintaining a compact footprint. A four-port feeding mechanism is integrated to support MIMO operation with improved channel diversity and reduced mutual coupling. The metasurface is realized using a 32×32 unit-cell array, where increasing the number of unit cells significantly improves gain due to enhanced aperture efficiency. The fabricated prototype is experimentally characterized, and the measured S-parameters demonstrate good impedance matching at the three operating frequencies, with acceptable agreement between simulation and measurement results. In addition, reduced mutual coupling between ports confirms effective MIMO performance across the three bands. Radiation characteristics are evaluated through both 2D and 3D patterns. The radiation patterns were measured for a single port at frequencies where the reflection coefficient shows optimal performance, specifically at 8.75 GHz, 9.75 GHz, and 10.5 GHz. At these frequencies, the antenna exhibits well-defined main lobes with symmetrical radiation characteristics, indicating stable radiation behavior across the operating band. The realized gain exceeds 12 dBi at all three frequencies, with a peak gain of approximately 13 dB, along with satisfactory directivity and radiation efficiency. The results confirm that array scaling is an effective approach for gain enhancement without significantly increasing system complexity. In addition, the proposed MIMO metasurface achieves excellent diversity performance with ECC values below 0.04, DG values close to 10 dB, balanced MEG characteristics, and CCL values below 4 bits/s/Hz. The obtained results confirm that the proposed metasurface is a promising candidate for compact high-performance X-band MIMO systems for radar and advanced wireless communication applications.