# Compact Sub 6 GHz Dual Band Twelve-Element MIMO Antenna for 5G Metal-Rimmed Smartphone Applications
## 3 Antenna Element Design Mechanism and Parameter Analysis  
To verify the operating principle of the antenna element and analyze the contribution paths of each mode within the operating band, we analyze the surface current distribution of the sub-6-GHz antenna element at the resonance frequency points in the low-frequency band (3450 MHz) and high-frequency band (4900 MHz). Figure 3 presents the surface current distributions of a single sub-6-GHz antenna element at 3450 MHz and 4900 MHz. At 3450 MHz, the current on the right side of the antenna is greater than that at 4900 MHz and is mainly distributed in the double-coupling loop comprising the feeding strip (\(\overline{\text{AB}}\)) coupled to shorting strip 1 (\(\overline{\text{CDI}}\)) and coupled to the upper edge of the metal frame on the right side. Therefore, the low-frequency resonance mode is mainly achieved through this right-side loop path. At 4900 MHz, the current is mainly distributed in the loop path formed by the feeding strip (\(\overline{\text{AB}}\)) and left shorting strip 2 (\(\overline{\text{FD2}}\)), which is coupled to the upper edge of the metal frame on the left side. Therefore, the high-frequency resonance mode is mainly achieved by this left-side loop path. To further understand the contribution of each mode, we analyze the proposed structure and two other designs denoted Case 1 and Case 2. Case 1 is the proposed antenna structure without shorting strip 1 (\(\overline{\text{CDI}}\)) and shorting strip 2 (\(\overline{\text{FD2}}\)), and Case 2 is the proposed antenna structure with shorting strip 1 (\(\overline{\text{CDI}}\)). Figure 4 presents a comparison of the simulated reflection coefficients for Case 1, Case 2, and the proposed structure. The Case 1  
Figure 3: Surface current distribution of an antenna element at 3450 MHz and 4900 MHz. Figure 2: Detailed structure of an antenna element in the sub-6-GHz 5G MIMO system. antenna structure does not produce any resonant modes in the low-frequency band but has a shallow resonant mode near 5000 MHz in the high-frequency band. When shorting strip 1 (\(\overline{\text{CD1}}\)) is added to the Case 1 antenna structure to form Case 2, a favorable resonant mode is generated in the low-frequency band. However, the high-frequency resonant mode matching is impaired. By adding a shorting strip 2 (\(\overline{\text{FD2}}\)) to form the proposed structure, the low-frequency mode can be preserved, and the high-frequency mode can be further optimized to cover the desired 4800-5000 MHz frequency band. The low-frequency resonant mode is mainly enhanced by the double-coupling loop path formed by the feeding strip (\(\overline{\text{AB}}\)) coupled to shorting strip 1 (\(\overline{\text{CD1}}\)) and then to the upper metal edge of the structure. The high-frequency mode is mainly enhanced by the left loop path formed by the feeding strip (\(\overline{\text{AB}}\)) and the left shorting strip 2 (\(\overline{\text{FD2}}\)) coupled to the metal frame at its upper edge. Three parameter analyses are conducted for the structure of Ant 1. First, a parameter analysis for length m of the right side of the metal frame with a 1 mm opening on the metal frame is conducted. The opening position is fixed, and grounded metal is added or removed at an appropriate clearance interval to increase or decrease the effective length of the right metal frame. Figure 5 presents a comparison of the simulated reflection coefficients as \(m\) varies. Increasing the right-side length of the metal frame from 9 to 11 mm generates a substantial downshift in the low-frequency resonant mode but does not affect the high-frequency resonant mode. This verifies that the low-frequency resonant mode is contributed to by the path on the right side of the metal frame opening. A parameter analysis for length \(n\) of the left side of the metal frame with a 1 mm opening is then conducted. The opening position is fixed, and grounded metal is added or removed as appropriate in the left clearance interval.