**5. Conclusions**

An ultrathin eight-port MIMO antenna functioning in 5G 3.4–3.6 GHz and 4.8–5 GHz is presented in this paper. The explored antenna element obtained a minimized dimension of 9 × 4.2 mm2, and the overall volume of the MIMO system was only 145 × <sup>70</sup> × 5 mm3. Two kinds of DBs were employed to attenuate the mutual coupling at 3.5 GHz. Besides the design stages of the proposed antenna, the role performed by the DBs were also studied to gain profound understanding of the decoupling mechanism. An application scenario of DHM was given to evaluate the robustness and practicability of the presented antenna. The measured −10 dB impedance band is able to contain the target bands entirely. The measured worst mutual coupling (14.5 dB) appeared in S23 around 3.5 GHz. Maximum radiating efficiency of 60% and 75% were obtained within the first and second bands, respectively. The computed results of indicators, such as the ECC (0.008), DG (9.978), TARC (10 dB), and ME (70%), have proved the excellent MIMO performance of the proposed antenna.

**Author Contributions:** Conceptualization, C.Z.; methodology, Z.C. and G.D.; investigation, Q.Y. and X.W.; writing—original draft preparation, C.Z.; writing—review and editing, X.S. and G.L.; supervision and funding acquisition, G.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was partly supported by the National Natural Science Foundation of China under grant 61671330, the Science and Technology Department of Zhejiang Province under grant LGG19F010009, and Wenzhou Municipal Science and Technology Program under grants C20170005 and 2018ZG019.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
