# Compact Sub 6 GHz Dual Band Twelve-Element MIMO Antenna for 5G Metal-Rimmed Smartphone Applications
## 4 Experiment and Measurement Results  
Figure 8 presents the overall configuration of the fabricated MIMO twelve-antenna system. Figure 9 displays the fabricated MIMO six-antenna unit and the antenna elements  
Figure 5: Simulated reflection coefficients for various \(m\) values. Figure 6: Simulated reflection coefficients for various \(n\) values. Figure 7: Simulated reflection coefficients for various \(d\) values. in detail. The antennas were fabricated in accordance with the dimensions presented in Figures 1 and 2 and were measured. Ansys HFSS (Version 18) software was used to conduct simulations. In the simulations, the desired threshold for the reflection coefficient was set to be less than -6 dB (equivalent to a voltage standing wave ratio of 3:1) [25; 26; 27]. This means that if the reflection coefficient of the antenna in both simulation and measurement was below -6 dB, it indicates that the desired antenna efficiency metrics could be easily achieved in actual tests. The transmission coefficient between the antennas was set as less than \(-10\) dB on the basis of the findings of previous studies [25; 26; 27]. Figure 10 presents the simulated and measured reflection coefficients of the MIMO twelve-antenna system. Both the simulated and measured reflection coefficients indicate that the system can achieve dual-band 5G sub-6-GHz operation at 3300-3600 MHz and 4800-5000 MHz. Both the low and high-frequency bands have one resonance mode, and the simulated and measured data are consistent. Moreover, both the simulated and measured reflection coefficients are smaller than \(-6\) dB (i.e., voltage standing wave ratio = 3:1). Figure 11 presents the simulated and measured transmission coefficients of the MIMO twelve-antenna system. For clarity, we only present the transmission coefficients of two adjacent antennas because adjacent antennas have the greatest mutual influence. The transmission coefficients for antenna pairs with greater separation are smaller [25]. The figure reveals that not all simulated transmission coefficients are smaller than \(-10\) dB in the frequency band (only \(-9.5\) dB or higher was achieved). However, the actual measurements demonstrate that transmission coefficients of -10 dB or better were achieved. In antenna design, measured data is often considered a more reliable basis than simulated data. This is because measured data reflect true performance under actual manufacturing and operating conditions. While simulations can provide valuable reference and initial assessments, they are still based on assumptions and idealized models. In Figures 10 and 11, measured results slightly differ from and improve compared to the simulated ones. These discrepancies and  
Figure 8: Fabricated MIMO twelve-antenna system. Figure 9: Fabricated MIMO six-antenna unit. causes could be due to the fact that the additional mini coaxial cables and IPEX-to-SMA adaptors used in measurement causing loss are not considered in simulation. Moreover, the parameter variation of the FR4 substrate and the imperfection in fabrication could also be part of the reasons for the discrepancies. Figure 11: (**a**) Simulated and (**b**) measured transmission coefficients of the MIMO twelve-antenna array. Figure 10: (**a**) Simulated and (**b**) measured reflection coefficients of the MIMO twelve-antenna array. For MIMO operations, certain performance parameters such as the ECC, mean effective gain (MEG), diversity gain (DG), and total active reflection coefficient (TARC) may also be of interest and can be examined. The ECC is a critical metric for evaluating MIMO antenna systems. Generally, an ECC of less than 0.5 is satisfactory for practical applications. Figure 12 presents the ECCs of the simulated and measured radiation pattern for the MIMO twelve-antenna system. For clarity, only the ECCs of adjacent antennas are presented because the influence is greatest for adjacent antennas. The simulated and measured ECCs within the frequency bands are all less than 0.41 and 0.38, respectively.