This research presents a new antenna array system with enhanced impedance bandwidth and reduced mutual coupling designed for sub-6 GHz smartphone applications. The antenna resonators are functioning within the frequency spectrum spanning from 3.3 to 4.2 GHz. The properties and performance of the antenna element, as well as the entire MIMO smartphone design, are meticulously evaluated using CST software. Through rigorous simulations, acceptable results have been discovered.
Figure 1(a) depicts the structural configuration of this multi-feed antenna system. The design features eight planar inverted F-antennas (PIFAs), with discrete feedings. They have been strategically positioned at left/right sides of the smartphone main board, yielding a concise overall footprint of 75 mm × 150 mm FR4. To easily integrate with phone circuits, the ground-plane and antenna resonators have been etched on the same side of the substrate. By addling modified T-shaped strips among the adjacent PIFA resonators, the operation bandwidth and isolation level of the closely spaced radiators have been significantly improved. Figure 2 (b) illustrates the S-parameters of the antenna pair, comparing performance with and without the inclusion of the T-shaped strip between the PIFA resonators. The results demonstrate that incorporating the T-shaped strip enhances the overall bandwidth and the isolation, ensuring more efficient operation of the closely spaced radiators. To theoretically analyze and validate of this enhancement, the simulated current distributions at 3.4/3.9 GHz for port 1 are shown in Fig. 1 (c). As shown, at the first resonance, the T-shaped strip structure, which constitutes the longer section of the design, demonstrates substantial current flow. In contrast, at the second resonance, the current is predominantly concentrated on the shorter PIFA arm, indicating its active contribution at higher frequencies. Figures 2 (a) and (b) illustrate the S-parameter results. As shown in Fig. 2 (a), the reflection coefficients (Snn) for all elements exhibit excellent matching over 3.3-4.2 GHz. Furthermore, as shown in Fig. 4 (b), the mutual coupling between the elements is reduced, with isolation levels (Snm) exceeding -16 dB. The 3D radiation patterns of the antenna elements, along with their corresponding gain values at 3.5 GHz are displayed in Fig. 2 (c). As shown, the design provides well-defined radiation with sufficient gain levels suitable for MIMO smartphone applications.