## 5 Feasibility and Measurement
The antenna geometry was further refined to enhance inter-port isolation and minimize the ECC values across the entire  
Figure 8: Photographs of the prototypic implementation stages of the sub-6 GHz metal frame-coupled mobile antenna. (**a**) Step 1: The initial two-port shared-coupling configuration with discrete feed connections along the metal frame. (**b**) Step 2: The refined two-port configuration. The pattern-tuned structure improves inter-port isolation and reduces the ECC. operating frequency. The final design parameters were as follows: top frame width W\({}_{1}\) = 45 mm, coupling gap C\({}_{\text{g}}\) = 0.6 mm, and ground and antenna widths (G\({}_{\text{W}}\), R\({}_{\text{W}}\)) 1.5 mm. The inter-element ground spacing G was 2 mm, and the radiating length (R\({}_{\text{L}}\)) along the frame was reduced to 5 mm to improve the field distribution. For each antenna port, the stub length and width (S\({}_{\text{L}}\), S\({}_{\text{W}}\)) were optimized to 3.5 and 6.5 mm, respectively, with a grounding length (G\({}_{\text{L}}\)) beneath the slot of 7 mm. These geometric modifications were aimed at fine-tuning the resonant characteristics and enhancing impedance matching over the targeted frequency bands. Additionally, a common grounding strategy was employed to improve further port isolation. Specifically, a central grounding screw was used to establish a robust shared ground reference between the PCB and the metal frame, thereby stabilizing the electromagnetic environment and reducing undesired coupling between adjacent antenna elements. The refined antenna layout enhanced inter-port isolation and substantially reduced the ECC across the n78 band, as validated by both the measured and simulated results of Figures 9 and 10. The two-step design approach enabled the systematic identification of structural dependencies. The method balanced the impedance bandwidth, spatial isolation, and MIMO correlations within the constraints of realistic smartphone integration. Such iteration highlights the need to co-optimize electromagnetic performance and mechanical implementation when seeking to meet the stringent requirements of sub-6 GHz 5G MIMO systems. Figure 9 shows the measured performance of the optimized two-port antenna after Step 2 [Figure 8a]. This addressed the limitations of the initial four-port shared coupling design, with a particular focus on improving the ECC and inter-port isolation. As shown in Figure 9a, both antenna ports (ANTs #1 and #2) achieve acceptable impedance matching, maintaining VSWR values below 3 across most of the n78 band (3.3-3.8 GHz). ANT #1 exhibits a broadband response with a primary resonance near 3.65 GHz and a secondary lobe around 4.6 GHz. ANT #2 maintains stable wideband matching across the 3.2-4.5 GHz range, ensuring robust sub-6 GHz performance. However, the measured isolation (\(|\)S\({}_{21}\)\(|\)) between the ports remains relatively low, being a minimum of \(-\)4.21 dB at 3.78 GHz, indicating strong mutual coupling. Figure 9b illustrates the corresponding radiation efficiency and ECC performance. Both antenna elements exhibit efficiencies greater than -6 dB across the bands, with peak values approaching \(-\)2 to \(-\)3 dB around 3.5 GHz. Despite the suboptimal isolation, the ECC values remain below 0.25 throughout the operational bands, with most of the n78 band exhibiting ECC values less than 0.2. This indicates that, although significant electromagnetic coupling remains, the far-field radiation patterns are sufficiently decorrelated to support the required MIMO diversity performance. These results confirm that the Step 2 configuration markedly improves the ECC compared to that of the original four-port design. However, further enhancements in isolation are necessary to fully meet the system-level MIMO requirements. Future work may incorporate decoupling techniques such as neutralization lines, grounded stubs, or parasitic isolation elements to suppress inter-port coupling further without compromising compact integration.