**1. Introduction**

Fifth-generation (5G) communication is burgeoning, demanding wireless devices with a transmission data rate as high as possible. Multiple input–multiple output (MIMO) technology possesses promising application prospects in improving the data rate. Recently, many sub-6 GHz 5G smartphone MIMO antennas have been developed [1–16], such as four-element MIMO antennas [2–5], eight-port smartphone antennas [6–10], and even twelve-element MIMO antennas [11,12]. One nonnegligible challenge encountered during the design process is the method to effectively weaken the mutual electromagnetic coupling between antenna elements in a MIMO antenna array. However, numerous decoupling mechanisms have been put forward, such as polarization diversity [9], defected ground structure (DGS) [12,13], decoupling branches [14], neutralization lines [15], and orthogonal mode [16]. More attention still needs to be focused on the decoupling design in the MIMO antenna array.

The usual height of the lateral side frame of a conventional smartphone's antenna [2,3,6,7] is 7 mm, which is not conducive to implementing future ultrathin smartphones. Some low-profile MIMO antennas for the 5G handsets have been proposed recently. In [17], a compact four-element MIMO antenna pair for 5G mobile was presented, integrating two antenna elements at a close distance of 1.2 mm. The designed antenna pair resonated precisely at 3.5 GHz. One more worthy mention is that the overall volume of the MIMO system was 150 × <sup>73</sup> × 6 mm3, which realized a 1 mm reduction in the height of the lateral side frame. Another self-decoupled four-element antenna pair [18] functioning in the 3.5 GHz band (3.4–3.6 GHz) with the same height of 6 mm has been presented, and the mutual coupling of the antenna pair was decreased to 16.5 dB. In [19], a low-profile, high-isolation eight-port MIMO antenna for the 5G handset was presented, and the height of the lateral sideboard was 5.3 mm.

**Citation:** Zhang, C.; Chen, Z.; Shi, X.; Yang, Q.; Dong, G.; Wei, X.; Liu, G. A Dual-Band Eight-Element MIMO Antenna Array for Future Ultrathin Mobile Terminals. *Micromachines* **2022**, *13*, 1267. https://doi.org/ 10.3390/mi13081267

Academic Editors: Lu Zhang, Xiaodan Pang and Prakash Pitchappa

Received: 10 July 2022 Accepted: 1 August 2022 Published: 6 August 2022

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This paper presents an ultrathin eight-port MIMO antenna working at 5G 3.4–3.6 GHz and 4.8–5 GHz frequency bands. The integral volume of the proposed antenna is only <sup>145</sup> × <sup>70</sup> × 5 mm3, which is thinner than other published 5G smartphone antennas. Eight antenna elements are manufactured along the inner face of two sideboards. Two decoupling branches (DB1 and DB2) are employed to attenuate the mutual coupling. The proposed antenna is fabricated and measured. The measured −10 dB impedance bands are 3.38–3.82 GHz and 4.75–5.13 GHz, which can fully contain two target bands. A measured lowest isolation (14.5 dB) emerged in S23 around 3.5 GHz. DHM mode is provided to assess practical application ability. ECC, DG, TARC, and ME are calculated to evaluate diversity performance.
