## 3 MIMO Configuration  
The proposed two-branches monopole antenna, loaded with the stub configuration is illustrated in Figure 5a, and its frequency response for varying values of \(d\) is depicted in Figure 5b. For different \(d\) values, as observed in Figure 5b, a stub separation of \(d\) = 11 mm was chosen as a compromise between bandwidth and port matching level. The resulting frequency response, depicted by the green curve in Figure 5b, exhibits a broader bandwidth extending from approximately 2.68 GHz to beyond 10 GHz. Critical coupling points occur at 4.64 GHz and 8.62 GHz, corresponding to \(S_{11}\) values of \(-\)10.34 dB and \(-\)10.64 dB, respectively. This straightforward design, which employs the reactive load induced by the T-shaped stub, offers ultra-wideband operation without increasing the complexity of the overall geometry or calculations. The compact antenna footprint of 14 \(\times\) 16 mm\({}^{2}\) is significantly smaller compared to a conventional strip monopole. Considering the free-space wavelength at the lower cutoff frequency, these dimensions correspond to 0.11\(\lambda_{0}\)\(\times\) 0.13 \(\lambda_{0}\), respectively. Based on the previous results, a four-element MIMO array is proposed which is illustrated in Figure 6a. An axial symmetry configuration was selected for the MIMO antenna design to maximize isolation among the radiated electric fields of the individual elements. Consequently, for a clearer representation of results, it is sufficient to illustrate the magnitude in dB of the \(S_{11}\), \(S_{21}\), and \(S_{31}\) parameters, as any other parameter combination would exhibit analogous behavior. The simulated \(S\)-parameter results are depicted in Figure 6b. Figure 5: (a) Two-monopole configuration, scale in mm; (b) Frequency response for different \(d\) values. The array footprint is 32 mm \(\times\) 32 mm, with the closest element spacing being 2 mm, as indicated in Figure 5(a). The impedance bandwidth of the array, depicted in Figure 5(b), extends from 2.8 GHz to 10.3 GHz, considering \(S_{11}=-10\) dB. Moreover, the electromagnetic isolation among the elements consistently is more than 18 dB, which is exceptional given the tight element spacing. This high level of isolation can be attributed not only to the orthogonal polarization of the elements but also to the stubs that act as an effective electromagnetic barrier. To demonstrate this hypothesis, a surface current analysis is carried out. The surface current distribution at different frequencies is illustrated in Figure 7, which clearly shows the reduced current induction among elements when antenna 1 (_A1_) is excited and also demonstrates how the electric length is increased since the surface current is coupled from the strip-monopoles to the thinner arm of the stub and fed back to the ground plane, making the trajectory longer, and therefore, obtaining a smaller lower-cutoff frequency. Also, Figure 6(a) shows the shielding of the electric field distribution and how its magnitude is reduced to other antennas when radiator 1 is excited. In this case, the analysis is shown at the middle band; however, the behavior is similar over the bandwidth, as depicted in Figure 6(b)-f.  
Once the simulated frequency response of the array, encompassing both bandwidth and isolation has been validated, the MIMO performance was evaluated. Then, the prototype based on the configuration of Figure 5(a) is built and its photograph is displayed in Figure 8.  
Figure 9 presents the measured \(S\)-parameters of the array, which show a bandwidth ranging from approximately 2.89 GHz to well around 12 GHz. Showing a small mismatching in the \(S_{44}\) parameter at 5 GHz; however, this result can be due to the construction process, since the other port matching parameters are below \(-10\) dB at this frequency. The isolation among the array elements is notably high, achieving values bigger than 18 dB in the lower frequency band and reaching values greater than 30 dB in the higher frequency band.