Nowadays, the MIMO can achieve fifth generation (5G) ultra-high capacity, but it is a great challenge for the smartphone antenna to achieve good isolation, high efficiency, and other performance in limited space. The paper designed and completed an eight-port dual-polarized high-efficiency shared-radiator antenna working in 3.5 GHz (3.4–3.6 GHz) for 5G mobile devices. The two antenna elements are regarded as one building block and share one radiator, and the size of one radiator is 17.1 × 17.1 mm2 (0.02 λ × 0.02 λ, where λ presents the free-space wavelength at 3.5 GHz). The MIMO system consists of four radiators, and the edge-to-edge distance between the radiators on the short side is 31.9 mm (0.038 λ), and the total size of the MIMO antenna system is 150 × 80 × 1.6 mm3 (0.176 λ × 0.094 λ × 0.0019 λ). The antenna uses an orthogonal placement of feed lines to produce dual polarization in the MIMO system, resulting in high isolation without introducing other decoupling structures. In addition, the reason for the high efficiency of the antenna is explained by the common mode (CM) and differential model (DM). Finally, the simulated results are as follows: the isolation is 14 dB; the total efficiency (TE) is 75–85%; the envelope correlation coefficient (ECC) is lower than 0.065; and the gain is 6.5 dB. The prototype is fabricated and tested: the isolation is better than 17 dB, the range of the measured TE is 60–75%, and the ECC is lower than 0.045. In addition, the influence of the human body model on the antenna are also discussed. Overall, the proposed MIMO antenna has a shared radiator with high isolation and high TE, and is more suitable for the current stage of 5G MIMO antenna technology. More importantly, the planar structure block is very simple to build and easy to fabricate on the substrate.

It is challenging to design a compact antenna pair with isolation better than 20 db and with robustness to antenna position. In this paper, a T-shaped dummy antenna is adopted to decouple the tightly arranged antenna pair for 5G mobile devices. Working in the 3.8 GHz of N77 band, the proposed antenna pair is with an impedance bandwidth of 280 MHz. Isolation of the proposed antenna pair is enhanced from 13 db to 30 db by adding the T-shaped dummy antenna, and it is with high robustness even though the antenna position is altered. The calculation process of the loading requirement of T-shaped dummy antenna is shown. By duplicating the proposed antenna pair, a 4×4 MIMO antenna with high isolation is proposed. The measured results show the MIMO antenna with isolation better than 31 db, and the average total efficiency is about 49%, and the Envelope Correlation Coefficient (ECC) is lower than 0.05 in the -6 db pass band. To the best of the authors’ knowledge, compared to most of the reported antenna pairs, the proposed 4××4 MIMO antenna is with the highest isolation while keeping a compact size.