# A Dual-Band 8-Antenna Array Design for 5G/WiFi 5 Metal-Frame Smartphone Applications
## 3 Design Process  
The design methodology for the antenna element is outlined in Figure 4a. Simulated reflection coefficients are presented in Figure 4b. The reflection coefficients for Case 1 and Case 2 indicate that the low-frequency resonance was generated by the inverted L-shaped strip atop the metal frame, while the high-frequency resonance was influenced by the C-shaped strip on the right side of the metal frame. Moreover, Case 3 demonstrates that the inverted L-shaped strip connected to the left rectangular strip effectively controlled the deviation between the two resonance points. Ultimately, the proposed antenna element covered the frequency range of 2.55-4.2 GHz in the lower band and 4.4-6 GHz in the higher band. Owing to disparities in radiation conditions and surface current distributions, key dimensions within the antenna yielded diverse effects on the antenna's frequency band. The influence of these critical parameters on the frequency band was examined based on actual simulation conditions. Non-essential parameters that did not significantly impact frequency variations were omitted and will not be discussed. To demonstrate the working effects of the proposed MIMO antenna, variations were introduced in the strip parameters L\({}_{1}\) and L\({}_{2}\) for each antenna element. Figure 5 displays the simulated S\({}_{11}\) parameter as it changed with different values of L\({}_{1}\), L\({}_{2}\), L\({}_{d}\), and L\({}_{5}\) while keeping other optimized parameters constant. A comparison of various L\({}_{1}\) and L\({}_{2}\) values revealed their distinct influence on low- and high-frequency resonances, respectively. As illustrated in Figure 5a, significant changes occurred in the low-frequency band of the antenna when altering the value of L\({}_{1}\). Specifically, the frequency decreased as the length of the L-shaped strip, L\({}_{1}\), increased. Conversely, modifying the length of the C-shaped strip line, L\({}_{2}\), led to notable fluctuations in the high-frequency band of the antenna, as depicted in Figure 5b. It is evident from Figure 5c,d that L\({}_{4}\) primarily affected the low-frequency band, while L\({}_{5}\) had a greater impact on the high-frequency band. Overall, compared to the influence of L\({}_{1}\) and L\({}_{2}\) on the antenna's operating frequency, the effect of L\({}_{4}\) and L\({}_{5}\) on the frequency band was relatively smaller when changing the same value. This observation is further supported by the vector current distribution diagram shown below. Consequently, the resonant frequency of the proposed antenna element can be independently adjusted, showcasing a unique characteristic that facilitates straightforward operation in engineering applications. Figure 4: (**a**) Four cases of the antenna element during the evolution process, and (**b**) simulated reflection coefficients. To enhance our understanding of the operational principle of the antenna element, we present the surface electric field at 3.5 GHz, 4.9 GHz, and 5.5 GHz in Figure 6. Initially, the surface electric field demonstrated its peak intensity at the two inverted L-shaped strips on the side-edge metal frame, with the current concentrated along the edge of the metal frame at 3.5 GHz. Subsequently, at 4.9 GHz, a robust electric field path was distributed along the bottom of the metal frame and the C-shaped strip. Finally, in the 5.5 GHz band, the maximum electric field values were observed at the top edge of the metal frame and the C-shaped strip. The current vector distribution of the antenna primarily concentrated on path 1 at 3.5 GHz, as depicted in Figure 6a. Moreover, both path 2 and path 3 are illustrated in Figure 6b,c, showcasing the direction of current flow in their respective operating bands. These findings signify that a portion of the metal frame, along with the ground, functions as the radiator of the antenna. Consequently, the resonances in the differential bands are independently associated with the fundamental modes of the distinct current paths.