## 3 MIMO Configuration
These results guarantee the low correlation between the electric fields of the radiators. The \(TARC\) analysis, presented in Figure 10c, compares the simulated and measured curves obtained from (8) for different values of \(\theta_{1}\). In this case, for \(N\) = 4, the \(TARC\) is influenced by three random phase angles, \(\theta_{1}\), \(\theta_{2}\), and \(\theta_{3}\), which vary continuously within the range of \(0^{\circ}\) to \(360^{\circ}\). To simplify the analysis, both \(\theta_{2}\) and \(\theta_{3}\) were fixed at \(0^{\circ}\), while \(\theta_{1}\) was varied. Both simulation and measurement results demonstrate that the \(TARC\) remains below \(-10\) dB from 2.7 GHz to 10.5 GHz and up to 11.7 GHz, respectively, confirming the wideband characteristics of the system. While both measured and simulated \(TARC\) curves align at the lowest cutoff frequency, indicating accurate initial design parameters, discrepancies within the specified bandwidth are likely attributed to fabrication tolerances. Minorvariations in substrate thickness, dielectric constant, and conductor dimensions, inherent in the manufacturing process, can significantly impact high-frequency performance. These variations, which are not always perfectly modeled in simulations, lead to deviations in the measured results. Finally, considering the MIMO metrics, the MEG ratio is calculated following (10) to (12), and the results are presented in Figure 8.6 According to [13], values of the MEG ratio around 1 dB are acceptable for MIMO applications, which threshold is valid for \(\pm\)3 dB. Regarding the radiation characteristics, Figure 12a,b provide a comparison of the simulated and measured gain patterns in the E- and H-planes at frequencies of 3 GHz, 5 GHz, and 10 GHz. The gain pattern plots reveal a quasi-omnidirectional radiation characteristic in the H-plane, while the E-plane exhibits pronounced peaks at \(0^{\circ}\) and \(270^{\circ}\). The T-shaped stub functions as a reflector, causing a reduction in the gain at \(270^{\circ}\) relative to \(90^{\circ}\). Specifically, peak gains of \(-2.3\) dB, \(-0.2\) dB, and 2.6 dB are attained at 3 GHz, 5 GHz, and 10 GHz, respectively. These findings are consistent with the general trend that miniaturization often entails trade-offs in terms of efficiency, gain, and bandwidth. However, in this design, while efficiency and bandwidth remain relatively unaffected, the reduction in size at lower frequencies results in a decrease in gain. On the other hand, applying (9) to calculate the \(DG\), the average value for this metric is around 9.9999, which implies that the spatial diversity gain employed in the array is very effective. Figure 8.7 Comparison of measured and simulated results; (**a**) _ECC S_-parameters method, (**b**) _ECC_ E-field method, (**c**) _TARC_. Finally, Table 1 presents a comparison with other four-port ultra-wideband MIMO antenna structures. The complexity * column in Table 1 is a subjective evaluation based on the authors' judgment of the radiator geometry and design intricacies. For example, the antenna in reference [15], inspired by the SARS-CoV-2 virus, is deemed complex due to its irregular structure and the numerous variables requiring optimization. Conversely, the design in reference [14] features simpler, replicable geometries. Based on these assumptions, the proposed antenna in this work exhibits very low complexity due to its regular and easily reproducible shapes based on conventional strip monopoles fed by a coplanar line. Table 1 also reveals that the proposed antenna has considerably smaller dimensions than the reported structures, occupying a much smaller portion of the wavelength, considering the lower cutoff frequency. Moreover, the proposed design demonstrates comparable or improved \(ECC\), \(TARC\), and \(DG\) performance compared to other configurations. Notably, the isolation among elements reaches values from 20 dB to close to 40 dB. Figure 8.8 Measured MEG ratio.