# Wideband Eight-Antenna Array Designs for 5G Smartphone Applications
## 3 Findings and Analysis  
The front and back views of the fabricated eight-antenna array prototype are depicted in Figure 5. Figure 6a,b display the simulated reflection coefficient (return loss) and transmission coefficient (isolation) of the eight-antenna array. For brevity, we only investigate Ant.1 to Ant.4. Figure 6a shows that the stacked 6 dB bandwidth (S11, S22, S33, and S44) adequately covers the bands-of-interest (3300-5925 MHz). Regarding the transmission coefficients (S21, S32, and S43), good isolation levels better than 10 dB were achieved. Figure 4: Simulated reflection coefficients: (**a**) tuning \(L_{1}\) and (**b**) tuning \(L_{2}\). Figure 5: Views of the fabricated prototype. Figure 7a presents the measured reflection coefficients (S11, S22, S33, and S44) of the proposed eight-antenna array. It demonstrates that the array covers the bands-of-interest with a 74.1% (3100-6750 MHz) stacked 6 dB bandwidth. Figure 7b shows that the measured transmission coefficients (S21, S32, and S43) closely match the simulated results in Figure 6b. Notably, the simulated and measured data of the proposed array are well validated, with slight differences likely due to fabrication tolerances. Figure 6: Simulated S-parameter diagram of proposed array: (**a**) reflection coefficient and (**b**) transmission coefficient. To further understand the excitation of the three loop modes in the proposed MLAE, Figure 8 illustrates the current distributions at 3.6, 4.0, and 5.1 GHz. The current path at 3.6 GHz represents a half-wavelength (0.5\(\lambda\) loop mode) distribution that flows from the feeding point F to the shorting point S, passing through a null point at the right upper corner of the P-shaped element. At 4.0 GHz, the current flows from the feeding point F to the P-shaped element (located on the left side of the MLAE), reaching a null point before returning to the shorting point S. This forms another half-wavelength (0.5\(\lambda\) loop mode) distribution. Lastly, at 5.1 GHz, the current path follows a conventional one-wavelength (1\(\lambda\) loop mode) distribution, extending from point F to point S via the connected bridge, with null points located at both ends of the P-shaped elements. Figure 7: Measured S-parameters of proposed array: (**a**) reflection coefficient and (**b**) transmission coefficient. Figure 9 is a photo of the proposed eight-antenna array placed in an anechoic chamber for measurement. The simulated and measured total efficiencies of the proposed array (Ant.1-Ant.4) are shown in Figure 10a,b, respectively. Ant.2 and Ant.3, positioned at the center of the array, exhibit slightly lower efficiencies due to the influence of adjacent antennas, compared to the outer elements (Ant.1-Ant.4). Despite this, the total efficiencies of all elements remain robust across the bands-of-interest, with simulated values ranging from 62% to 85% and measured values from 51% to 84%. These performance levels meet the typical standards (>40%) required for practical smartphone applications. Figure 8: Current distributions of MLAE at three resonant loop modes. Figure 9: Proposed eight-antenna array placed in an anechoic chamber for measurement. Figure 11a-c display the normalized radiation patterns of the proposed array (Ant.1-Ant.4) at 3.6, 4.0, and 5.1 GHz. The radiation patterns of Ant.1 and Ant.4 are very similar, as are those of Ant.2 and Ant.3. Despite slight variations across the three resonant modes, the main beam directions of the patterns are consistently oriented towards the \(-\)X and \(+\)Z directions. This orientation facilitates excellent envelope correlation coefficient (ECC) values, enhancing the CC and overall performance of the MIMO system when employing this eight-antenna array. Figure 10: Total efficiency of eight-antenna array: (**a**) simulation and (**b**) measurement.