# A Design for a Wide-Band Antenna Pair Applied for Mobile Terminals at the Sub-6 GHz Band
## 1 Introduction
In [21], eight antenna elements were evenly placed vertically on the smartphone bezel. Antenna element spacings of 32.375 mm allowed for less than \(-12\) dB isolation. In [22], a quad-port MIMO system with four L-shaped dual-band antennas was proposed. The isolation was lower than \(-16.5\) dB, which was implemented by placing different antennas at a distance of more than one wavelength. An eight-port MIMO system operating at 3.3-3.7 GHz, with an isolation value of less than \(-15\) dB, was proposed by placing eight \(4.6\times 5.6\) mm\({}^{2}\) antenna elements with equal spacing on the entire bezel [23]. An eight-port MIMO system consisting of loop antennas, with isolation lower than \(-16\) dB, was designed by the authors of [24]. An ultra-wide band of 3.3-5.6 GHz can be obtained, but the single antenna element size is \(17.5\times 7.5\) mm\({}^{2}\). Although the above MIMO scheme radiation characteristics meet the requirements of the 5G terminal radiation characteristics, the structures of the antenna systems are not compact enough, leaving limited space for 4G, WiFi, and other modules. In order to keep the balance between physical size and the coupling effect, research on decoupled antenna pairs has increased. Those decoupled antenna pairs using novel structures combine mainly the radiation elements and decoupled models to attain reliable isolation at a certain frequency range. In [25], the authors constructed a compact MIMO system with a high isolation value of \(-24\) dB at a central operating frequency. The chip capacitor was applied between two compact antenna pairs to obtain reliable isolated results but it is only useful for the limitation band. In [26], the authors designed tightly arranged four-element antenna elements with a high isolation value of \(-11.6\) dB. Similarly, inductors and capacitors served as decoupling elements to obtain radiation uncorrelation, which also limited the operating bandwidth of a single frequency at 3.4-3.6 GHz was effective. In [27], the proposed wide-band decoupled antenna pairs covered N77/N78/N79 bands, with isolation better than \(-15\) dB. The DGS on the ground plane and the PE between two elements as decoupled roles was applied. However, to meet the impedance matching, long patch stubs were added to the radiation antennas, and the total size of the antenna pairs was more than \(34\times 6\) mm\({}^{2}\). In this article, a compact planar antenna pair with UWB and high isolation is proposed. The total size of the antenna pairs is \(23\times 6\) mm\({}^{2}\), and the inner distance of two adjacent radiation elements with antenna pairs is only 2 mm. The simulated results for the single antenna pair in the \(75\times 35\) mm\({}^{2}\) ground plane indicate that the \(-6\) dB impedance bandwidth is 54% (3.3 GHz-5.9 GHz), which can cover 2600 MHz of bandwidth of the 5G Sub-6 GHz band. Due to the compact arrangement of the antenna elements, the mutual coupling effect is serious. Therefore, a strip-DGS is added in the middle of the antenna pairs, meaning that a good isolation of lower than \(-13\) dB is realized. Moreover, the eight-port MIMO array in the \(150\times 70\) mm\({}^{2}\) ground plane comprising the above antenna module is simulated and measured. The results show good isolation, low ECC, and good total efficiency in the context of mobile terminal applications. The radiation performance of the proposed MIMO system is then simulated and measured.