## 3 Results  
The antenna electrical performance was measured using 50 \(\Omega\) SMA connectors. The reflection coefficients (simulated and measured) of the designed UWB MIMO antenna at different ports (S\({}_{11}\), S\({}_{22}\), S\({}_{33}\), S\({}_{44}\)) are shown in Figure 7. While performing measurements at one resonating element, the remaining antenna ports were matched with 50 \(\Omega\) loads. The HCSRR embedded in the antenna radiator eliminated the Wi-MAX and WLAN frequencies. The frequency of the elimination bands can be altered by varying dimensions of the HCSRR. The mutual coupling among different resonating elements of the MIMO antenna is illustrated in Figure 8a,b. The inter-element coupling was decreased through an orthogonal arrangement of the resonating elements and by introducing metal strips between their ground planes. Isolation greater than 18 dB was attained for impedance bandwidth range. As displayed in Figure 9, a 7 dBi peak gain was realized in the proposed UWB antenna. A sudden fall in the gain was observed at dual-band rejection frequencies. Figure 8: \(Cont.\)  
Figure 7: S-parameters (simulated and measured) of the MIMO antenna. The surface-current behaviors at notch frequencies 3.5 GHz and 5.5 GHz are presented in Figure 10a,b, correspondingly, here, all the four elements were excited concurrently. The maximum concentration of current (highlighted in red color) was observed near to the boundary of the outer hexagonal split-ring (Figure 10a), which resulted in Wi-MAX band rejection. Similarly, high current concentration near the boundaries of the inner hexagonal split-ring (Figure 10b), marked the rejection of the WLAN band. ECC can be used to study the coupling effect between adjacent elements of the antenna. The given expression can be used to calculate ECC values between the first and second ports of a multi-port MIMO system [21]. \[\rho_{e}=\frac{\left|S_{11}^{*}S_{12}+\ S_{21}^{*}S_{22}+S_{13}^{*}S_{32}+\ S_{14}^{*}S_{42}\right|^{2}}{ \left(1-\left|S_{11}\right|^{2}-\left|S_{21}\right|^{2}-\left|S_{31}\right|^{2 }-\left|S_{41}\right|^{2}\right)\left(1-\left|S_{12}\right|^{2}-\left|S_{22} \right|^{2}-\left|S_{32}\right|^{2}-\left|S_{42}\right|^{2}\right)} \tag{5}\]  
Figure 8: S-parameters (simulated and measured) of the MIMO antenna at (**a**) port-1; (**b**) other ports. Figure 9: Simulated and measured gain of the proposed antenna. In a similar way, the ECC among remaining antenna elements can also be determined. Figure 11 displays the ECC curves among different elements. ECC was observed to remain below 0.07 for the entire UWB band. Figure 12 illustrates the co-polar and cross-polar radiation characteristics (simulated and experimental) of the designed MIMO antenna at frequencies 5 GHz, 9 GHz, and 14 GHz. A difference higher than 15 dB was seen among the co-polar and cross-polar pattern curves (in E- and H-planes), which shows the stable functioning of the designed antenna. The co-polar patterns of the E-plane displayed bi-directional behavior while the H-plane co-polar patterns possessed omnidirectional behavior. The simulated and experimental outcomes were found in close proximity. The variations observed between the measured and simulated outcomes were due to high loss tangent, antenna fabrication errors, and soldering of the SMA connectors. Table 2 lists the comparison of the presented dual-band notched UWB MIMO antenna to other existing UWB MIMO antennas in the literature. The comparison table highlights that the designed antenna has numerous advantages over the previous reported notched-band antennas [10; 11; 12; 13; 14; 15; 16; 17; 18], with reference to size, bandwidth ratio, inter-element isolation, and the number of radiating patches. The three metal ring-based decoupling technique used for reducing mutual coupling is simple and effective. In the presented MIMO antenna, the notch frequencies (3.5 GHz and 5.5 GHz) were removed  
Figure 11: Envelope correlation coefficient (ECC) of the proposed four-port ultra-wideband (UWB) MIMO antenna.