## 5 Conclusions  
A compact four-port MIMO antenna is proposed, utilizing a radiator configuration consisting of two conventional strip-monopoles. To achieve ultra-wideband operation, a T-shaped stub is introduced, extending from the top of the monopole array to the bottom of one end of the ground plane. The stub's strategic placement and dimensions effectively stabilize the real component of the input impedance and compensate for the reactive component, resulting in a near-zero reactance value over a broad frequency range. This design approach enables the overlapping of the two monopoles' resonances and impedance compensation, leading to a substantially increased bandwidth. The proposed antenna's versatile and simple structure exhibits excellent performance in a 4-element array with axial symmetry, demonstrating outstanding _ECC_, _TARC_, _DG_, and isolation between ports, even with closely spaced radiators. The overall array dimensions are approximately 0.083 \(\lambda_{0}^{2}\), considering the lower cutoff frequency. The T-shaped stub turns out to be a powerful tool to \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Reference** & \begin{tabular}{c} **Area [mm\({}^{2}\)]** \\ **(\(\lambda_{0}\)\({}^{2}\))** \\ \end{tabular} & \begin{tabular}{c} **BW** \\ **[GHz]** \\ \end{tabular} & _ECC_ & _DG_ & \
\begin{tabular}{c} **_TARC_** \\ **(\(\leftarrow\)\(10\) dB)** \\ \end{tabular} &
\begin{tabular}{c} **Isolation** \\ **[dB]** \\ \end{tabular} & **Complexity \({}^{*}\)** \\ \hline
[14] &
\begin{tabular}{c} 44 \(\times\) 55 \\ (0.47 \(\lambda_{0}\)\(\times\) 0.58 \(\lambda_{0}\)) \\ \end{tabular} & 3.19–9.3 & \textless{}0.02 & NA & \textasci{}3–9 GHz & 20 to 50 & M \\ \hline
[15] &
\begin{tabular}{c} 45 \(\times\) 45 \\ (0.675 \(\lambda_{0}\)\(\times\) 0.675 \(\lambda_{0}\)) \\ \end{tabular} & 4.5–16.5 & \textless{}0.002 & \textasci{}10 & \textasci{}4–16 GHz & 20 to 40 & H \\ \hline
[16] &
\begin{tabular}{c} 40 \(\times\) 40 \\ (0.42 \(\lambda_{0}\)\(\times\) 0.42 \(\lambda_{0}\)) \\ \end{tabular} & 3.2–13.4 & \textless{}0.8 & 9.90 & NA & \textgreater{}20 & M \\ \hline
[17] &
\begin{tabular}{c} 92 \(\times\) 92 \\ (0.61 \(\lambda_{0}\)\(\times\) 0.61 \(\lambda_{0}\)) \\ \end{tabular} & 2–14 & \textless{}0.36 & \textgreater{}9.6 & Fails to meet & \textgreater{}15 & L \\ \hline
[18] &
\begin{tabular}{c} 50 \(\times\) 50 \\ (0.4 \(\lambda_{0}\)\(\times\) 0.4 \(\lambda_{0}\)) \\ \end{tabular} & 2.4–18 & \textless{}0.0002 & \textgreater{}9.9 & 2.4–18 GHz & \textgreater{}25 & H \\ \hline
[19] & \
\begin{tabular}{c} 43 \(\times\) 43 \\ (0.3 \(\lambda_{0}\)\(\times\) 0.3 \(\lambda_{0}\)) \\ \end{tabular} & 2.15–20 & \textless{}0.01 & NA & 2.1–20 GHz & \textgreater{}20 & H \\ \hline This work &
\begin{tabular}{c} 32 \(\times\) 32 \\ (0.28 \(\lambda_{0}\)\(\times\) 0.28 \(\lambda_{0}\)) \\ \end{tabular} & 2.89–12 & \textless{}0.002 & 9.99 & 2.7–11.5 GHz & \textgreater{}18 & VL \\ \hline \hline \end{tabular}  
\({}^{*}\)M (Medium), H (High), L (Low), VL (Very Low).  
\end{table} 
Table 1: Comparison of state-of-the-art 4-port UWB MIMO antennas. couple and expand the bandwidth of antennas like monopoles. The result is a reduced-size antenna with applications in personal communications and communications. Conceptualization, J.A.T.-M. and H.J.-A. ; methodology, J.A.T.-M. and R.L.-M.; software, R.F.-L.; validation, E.F.-A., [PERSON]-M. and R.G.-V.; formal analysis, [PERSON]-M. and H.J.-A., investigation, J.A.T. [PERSON] and [PERSON]; resources, [PERSON], [PERSON] and [PERSON]; data curation, [PERSON]; writing--original draft preparation, [PERSON]; writing--reviewing and editing, [PERSON] and [PERSON]; visualization, [PERSON]; supervision, [PERSON] and [PERSON]; project administration, [PERSON]; funding acquisition, [PERSON]M. All authors have read and agreed to the published version of the manuscript. This research was funded by Instituto Politecnico Nacional, grant number 20250817. Not applicable.