## 4 Design of the Proposed UWB-MIMO Antenna
For the frequencies 5 GHz and 8.5 GHz, it is the edges of the pentagonal patch that are in front of the partial ground that radiate, and a greater current distribution is observed for 8.5 GHz over the entire parasitic element. For higher frequencies, the radiation is obtained by the edges of the radiator  
Figure 7: Surface current distributions in different parts of the MIMO antenna for \(f=3.1\) GHz, \(f=3.5\) GHz, \(f=5\) GHz, \(f=8.5\) GHz, \(f=10\) GHz, and \(f=10.6\) GHz. located far from the CSRRs. Isolation in these cases is less important, as shown by the coupling parameter in Figure 5.  
To highlight the effect of the introduction of CSRR structures in bandwidth enhancement and mutual-coupling reduction, the simulated reflection coefficients S11 and S22 and the coupling coefficients S12 and S12 are plotted for both cases (with and without the introduction of CSRR structures) in Figure 8a,b. Indeed, in the case of the MIMO antenna without CSRR structures, the achieved frequency bandwidth spreads from 3.4 GHz to 11.39 GHz. With the introduction of the CSRR structures, the achieved frequency bandwidth spreads from 3.01 GHz to 12.34 GHz. Concerning the isolation, illustrated in Figure 8b, the introduction of the CSRR structures increased the frequency band for which the coupling coefficient was greater than 20 dB from 3.46 GHz to 2.89 GHz. For the overall frequency bandwidth, the MIMO antenna with CSRR structures presents greater isolation than the MIMO antenna without CSRRs, except at 5 GHZ. This is confirmed by the current distributions, illustrated in Figure 9, plotted for the frequencies 3.1, 3.5, 5, and 8.5 GHz. The setup for measuring the radiation pattern is shown in Figure 10. Figure 10a shows the block diagram of this setup, while Figure 10b shows the layout inside an anechoic chamber. In this scheme, the horn antenna acts as the transmitter, while the design under test (AUT) functions as the receiver. Both antennas are connected to the VNA, which provides the necessary power to the horn and collects power from the AUT. The resulting data are then fed to the software, which generates the corresponding radiation pattern. The normalized two-dimensional radiation patterns of the UWB-MIMO antenna in the \(\varphi=0\)  and \(\theta=90^{\circ}\) planes are depicted in Figure 11, corresponding to the excitation of Port1 at frequencies of 3.1 GHz, 7.5 GHz, and 10.6 GHz. The observed results exhibit a strong level of agreement. The radiation pattern of the proposed antenna looks like the horizontal dipole radiation pattern, it is omnidirectional, and the \(\varphi=0\) plane corresponds to the H-plane and the \(\theta=90^{\circ}\) plane corresponds to the E-plane. A slight inclination is observed for 7.5 GHz and 10.6 GHz. Figure 8: Simulated results of S parameters of the proposed antenna with and without CSRR structures: (a) reflection coefficients S\({}_{11}\)/S\({}_{22}\); (b) coupling coefficients S\({}_{12}\)/S\({}_{21}\). Figure 12 depicts the simulated and measured results for the peak gain and radiation efficiency of the UWB-MIMO antenna. The peak gain exhibits a range of from \(-1\) dBi to \(6\,\mathrm{dBi}\), and the efficiency ranges from \(77.3\%\) to \(95.3\%\), within the desired FCC UWB frequency range. The simulation was carried out without introducing the SMA connector; this is the cause of the differences observed between the simulations and measurements. The introduction of this port into the manufactured prototype caused additional ohmic losses, leading to the reduction in the antenna efficiency and, consequently, its gain. Table 1.0 Radiation pattern measurement setup: (**a**) block diagram, and (**b**) layout inside anechoic chamber. Table 1.1 Surface current distributions in different parts of the MIMO antenna without CSRR structures for \(f=3.1\,\mathrm{GHz}\), \(f=3.5\,\mathrm{GHz}\), \(f=5\,\mathrm{GHz}\), and \(f=8.5\,\mathrm{GHz}\).