# Compact Sub 6 GHz Dual Band Twelve-Element MIMO Antenna for 5G Metal-Rimmed Smartphone Applications  
[PERSON]  
1 Electrical and Electronic Engineering Department, Chung Cheng Institute of Technology, National Defense University, Taoyuan 335, Taiwan; [EMAIL_ADDRESS] (C.-C.L. );  
[EMAIL_ADDRESS] (S.-C.C.) [PERSON]  
1 Electrical and Electronic Engineering Department, Chung Cheng Institute of Technology, National Defense University, Taoyuan 335, Taiwan; [EMAIL_ADDRESS] (C.-C.L. );  
[EMAIL_ADDRESS] (S.-C.C.) [PERSON]  
1 Electrical and Electronic Engineering Department, Chung Cheng Institute of Technology, National Defense University, Taoyuan 335, Taiwan; [EMAIL_ADDRESS] (C.-C.L. );  
[EMAIL_ADDRESS] (S.-C.C.) [PERSON]  
2 Electrical Engineering Department, National Yunlin University of Science and Technology, Douluition 640, Taiwan; [EMAIL_ADDRESS] (Correspondence: [EMAIL_ADDRESS] 2023 by the authors. Liconse MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons. Smartphones with metal frames are considered to have pleasing textures and aesthetics. Moreover, they are durable and can effectively dissipate heat. Some studies [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18] have designed MIMO antennas for smartphones with metal frames. In other studies, multi-antenna systems have been configured on a mobile phone device with the antennas being placed at appropriate distances from each other to optimize the isolation between the antennas [8; 9; 10; 11; 12; 13]. The distances between neighboring antenna pairs are approximately 12.5-31 mm. The interaction between the two antenna elements is greatly affected by the surface current of the metal frame. To address this, the authors of [8] created a slot on a metal frame to physically separate two antenna elements and etched a groove on the ground to counteract the coupled currents. In [9], the authors proposed two types of elements, namely an inverted H-shaped slot antenna and a modified top-loaded monopole, to achieve low mutual coupling. The radiation mode of the inverted H-shaped slot antenna could not be excited by the modified top-loaded monopole, resulting in high isolation between the two antennas. The proposed antenna in [10] comprised E-shaped monopole elements that were fed by microstrip lines, thin rectangular slots in the ground plane, and multiple slots and slits in the metal frame. This design improved the mutual coupling characteristics between each antenna element without requiring a decoupling structure. In [11], the authors developed a wideband antenna pair designed for 5G mobile phones. Each element of the antenna pair features an open slot and a coupled feeding line. Grounding branches were used to minimize the mutual coupling between the two elements. In [13], the antenna elements in the authors' design follow a slot antenna configuration, consisting of an L-shaped open slot and a 50 \(\Omega\) microstrip feedline. To achieve optimal impedance matching in the higher frequency band, the authors incorporate a tuning stub loaded onto the feedline. This approach ensures excellent impedance matching and enhances the overall performance of the antenna system. Although mechanisms for achieving low mutual coupling effects have been proposed in [8; 9; 10; 11; 12; 13], the designs of these studies all require a distance to be present between antenna pairs to maintain good isolation. To design decoupling mechanisms or achieve impedance matching, integrated metal-frame MIMO antennas for smartphones have been connected with components such as capacitors or inductors [14; 15; 16; 17; 18]. In [14], the authors presented a MIMO antenna for a full-screen metal-frame smartphone that covered the low-band (824-960 MHz), mid-band (1710-2690 and 3300-3800 MHz), and high-band (5800-5825 MHz) frequencies. The low-band and mid-band antennas added capacitors and inductors as a matching circuit for impedance adjustment.