# Dual-Band MIMO Antenna for 5G/WLAN Mobile Terminals
## 4 Experimental Results and Discussion  
To validate the proposed design, an antenna prototype was fabricated with the optimized dimensions listed in Figure 1. Figure 9 shows the photograph of the measurement experiment, including the vector network analyzer (VNA) instrument, the microwave anechoic chamber, and the fabricated antenna prototype.  
Figure 8: Simulated 3D radiation patterns when the device is held in single hand at: (**a**) 3.5 GHz; (**b**) 5.5 GHz.  
Figure 7: Simulated electric field distributions when the device is held in single hand at: (**a**) 3.5 GHz; (**b**) 5.5 GHz.  
The S-parameters are measured by Keysight Vector Network Analyzer N5224A. The measured S-parameters are shown in Figure 10. The \(-10\) dB impedance bandwidth (33-3.84 GHz and 4.61-6.8 GHz) can completely cover 5G New Radio N78 (3.3-3.8 GHz), 5G China Band N79 (4.8-5 GHz), and IEEE 802.11 ac (5.15-5.35 GHz, 5.725-5.85 GHz). A slight frequency shift can be observed, which may be due to several factors such as soldering-related effects and limitation of the milling machine. The measured isolation between each port is better than 15 dB.  
Figure 10: Measured coefficients of the proposed 4 \(\times\) 4 MIMO system: (**a**) reflection coefficient; (**b**) transmission coefficient.  
Figure 9: Photo of measurement experiment. (**a**) Vector network analyzer (VNA) instrument; (**b**) microwave anechoic chamber; (**c**) fabricated antenna prototype.  
As depicted in Figure 11, the measured total efficiencies are better than 70%.  
Figure 12 shows that the measured realized peak gain are better than 4.2 dBi.  
Figure 13 shows the measured reflection and transmission coefficients when the device is held in a single hand. The reason for the low frequency variation is the current distribution at the 3.5 GHz towards the inner direction of the substrate, which is disturbed by hand. However, the \(-6\) dB impedance bandwidth is still covering 3.3-3.8 GHz and the isolation is better than 14.2 dB.  
Figure 11: Measured total efficiencies of the proposed 4 \(\times\) 4 MIMO system.  
Figure 12: Measured realized peak gains of the proposed 4 \(\times\) 4 MIMO system.  
Figure 14 shows that the measured reflection coefficient and transmission coefficient when holding the device with both hands. Different from single-hand holding, dual-hand holding has less effect on S-parameter in the lower frequency band. The \(-9\) dB impedance bandwidth can cover 3.3-3.8 GHz and the isolation is better than 14.4 dB.  
In this study, the imaginary part and real part of the S-parameters are measured by VNA, and the calculated ECC formula for the proposed eight-antenna MIMO system is as follows:  
\[\rho_{\mathrm{ij}}=\frac{\left|\mathrm{S}_{\mathrm{ii}}^{*}\mathrm{S}_{ \mathrm{ij}}+\mathrm{S}_{\mathrm{j}}^{*}\mathrm{S}_{\mathrm{ji}}\right|^{2}}{( 1-(\left|\mathrm{S}_{\mathrm{ii}}\right|^{2}+\left|\mathrm{S}_{\mathrm{ji}} \right|^{2}))(1-(\left|\mathrm{S}_{\mathrm{jj}}\right|^{2}+\left|\mathrm{S}_{ \mathrm{ji}}\right|^{2}))} \tag{1}\]  
The calculated envelope correlation coefficient (ECC) of the proposed MIMO antenna system is shown in Figure 15. The values of the ECC in the operating frequency bands are smaller than 0.02.  
Figure 14: Measured coefficients when the device is held in dual hands: (a) reflection coefficient; (b) transmission coefficient.  
Figure 13: Measured coefficients when the device is held in single hand: (a) reflection coefficient; (b) transmission coefficient.  
The radiation patterns of Ant. 1 at 3.5 GHz and 5.5 GHz are shown in Figure 16. The measured co-pol and cross-pol are represented by solid and dash lines. The left column is the xoy plane, and the right column is the yoz plane.  
Table 1 shows a comparison between the presented antenna and other reported smartphone 5G MIMO antennas.  
Figure 16: Radiation patterns of the proposed \(4\times 4\) MIMO system: (**a**) 3.5 GHz; (**b**) 5.5 GHz.  
Figure 15: Calculated ECC of the proposed \(4\times 4\) MIMO system.