An eight-element multiple-input multiple-output (MIMO) frame antenna array in the 3.5 GHz band (3400-3600 MHz) for 5G mobile terminal systems was presented. By using the adjacent grounding and electromagnetic coupling feeding technology, the loop antenna element could generate two resonant frequencies, thus effectively expanding its bandwidth. By adopting double-sided parallel strip line (DSPSL) technology, the electromagnetic coupling inside the loop antenna could be adjusted, and the size of the loop antenna could be effectively reduced so that the MIMO antenna array could obtain a low-profile structure. The total size of the MIMO array was 150 mm × 75 mm × 5.3 mm. Without additional isolation measures, the measured -6 dB impedance bandwidth (BW) was 3400-3660 MHz, and the minimum isolation between antenna elements was better than -20 dB. The proposed antenna was expected to be applied to 5G mobile terminals based on its low-profile, high-isolated characteristics, and good MIMO performance.
