## 4 Design of the Proposed UWB-MIMO Antenna  
The printed geometry and design parameters of the proposed monopole-based MIMO antenna are illustrated in Figure 1. The optimized geometrical design parameters are given in Table 1. This antenna consists of two 50 \(\Omega\) microstrip-fed pentagonal radiators associated with two rectangular parasitic elements placed symmetrically on an FR4 dielectric substrate with a thickness (h) of1.6 mm, a relative dielectric constant (\(\varepsilon_{\mathrm{r}}\)) of 4.3, and a loss tangent (tan \(\delta\)) of 0.025. Three CSRR structures are etched into the middle of the L-shaped defective ground plane for bandwidth enhancement and coupling reduction. The proposed design has a compact overall size of 30 \(\times\) 20 \(\times\) 1.6 mm\({}^{3}\).Ant-c, illustrated in Figure 2, is used to form a two-element MIMO antenna, with a compact overall size of 30 \(\times\) 20 \(\times\) 1.6 mm\({}^{3}\). To conduct the S-parameter measurements, the design prototype is connected to the PNA-X Vector Network Analyzer (VNA) device using appropriate RF/microwave cables and connectors. After calibrating the VNA, the measurement frequency range and sweep resolution are defined, and the measurement format (S11, S21, S12, S22) is selected based  
Figure 4: Effects of different values of CSRR on antenna bandwidth and impedance matching: (**a**) length \(l_{1}\); (**b**) length \(l_{2}\) and parasitic element; (**c**) length \(l_{p}\); and (**d**) width \(w_{p}\). on the related requirements. This systematic process ensures an accurate and reliable S-parameter characterization of the RF and microwave components. Figure 5 illustrates the simulated and measured antenna S parameters. It can be seen that the simulated reflection coefficients S11 and S22 of the two ports are identical due to the system symmetry. We ascertained the same for the coupling coefficients S12 and S21. The reported measured values of S11 and S12 are achieved by feeding Port1 while Port2 is terminated via a matched 50-ohm load. The differences observed between the simulated and measured values are due to the introduction of SMA connectors and cables in the measurements. It can be seen that the relative error between the S parameters associated with the two ports does not exceed 5% and 2% for the\(\Delta\)Sii/Sii and \(\Delta\)Sij/Sij, respectively. The \(\Delta\)Sii/Sii is slightly remarkable for frequencies between 9 and 12 GHz compared to those in the 2-8 GHz band, which may be attributed to measurement errors, such as the placement and soldering of the SMA port, factors that can justify this small discrepancy. The FCC bandwidth calculated for S11 under \(-\)10 dB was achieved, and the isolation between the adjacent antenna radiating elements, illustrated by the coupling coefficient S12, was greater than 22 dB in the overall frequency bandwidth. Figure 6 shows a prototype of the fabricated MIMO antenna built to the aforementioned specifications. Figure 5: Simulated and measured results of S parameters: (**a**) S11 and S22, and (**b**) S12 and S21. Figure 6: Fabricated prototype of the proposed UWB-MIMO antenna: (**a**) top layout, and (**b**) bottom layout. The surface current distribution is simulated by exciting Port1 while Port2 is terminated with a 50 \(\Omega\) load. The obtained surface current distributions for 3.1 GHz, 3.5 GHz, 5 GHz, 8.5 GHz, 10 GHz, and 10.6 GHz are illustrated in Figure 7. These frequencies were chosen to highlight the radiation and isolation mechanisms. For low frequencies, the maximum current distribution is observed in the first two CSRR structures, the part of the defected ground located between the CSRRs and the radiating element, as well as the edges of the radiator located at the sides of the CSRR structures. A weak current distribution is observed in one edge of the parasitic element, and a perfect isolation is observed for the frequency 3.5 GHz.