## 1 Introduction  
Current telecommunication systems must meet the demand for high data rates, enabling end-user devices to access services such as multimedia, streaming, internet browsing, video calls, and many others. To achieve this, one of the most viable options is to optimize channel capacity, either by increasing bandwidth or by improving the signal-to-noise ratio (SNR). For the former, due to the saturation of the radio spectrum below 10 GHz, millimeter-wave bands are required. However, this leads to higher transmission losses and adverse propagation effects, necessitating the deployment of base stations within distances of around 100 m, which would significantly increase costs for operators. On the other hand, increasing the SNR is not a straightforward solution, as it requires increasing signal power, which also raises system costs. Nevertheless, to enhance SNR, antenna arrays are proposed to shape the radiation pattern and exploit multipath propagation as an advantage rather than a disadvantage. Multiple proposals have been made for this purpose, with multiple-input multiple-output (MIMO) antenna arrays gaining significant traction. In the case of a base station MIMO array, the limitations are fewer compared to personal devices due to the available space. In handsets, MIMO arrays must be placed in close proximity due to spatial constraints. This close arrangement leads to performance degradation of the array. As a result, these devices must meet specific requirements such as low electric field correlation, high isolation among radiators, and suitable bandwidths for the intended application. There is a wide variety of radiators that can be employed in MIMO arrays, ranging from the most basic, such as a monopole, to more sophisticated structures. However, many of these structures exhibit high-quality factors, which may limit their bandwidth for specific applications. Consequently, various approaches have been proposed to enhance this parameter, for example, ref. [1] introduces the use of a mushroom-type Electronic Band-Gap Structure (EBG) array at the backplane of a patch antenna to improve bandwidth. Ref. [2] presents a method that combines reactive impedance surfaces with metasurfaces to reduce the size and simultaneously increase the bandwidth of a microstrip antenna. In [3], the authors also employ a low-profile metasurface to enhance the bandwidth of a reflectarray antenna. Nevertheless, bandwidth enhancement techniques for radiators are not limited to the use of metasurfaces; reactive loads can also be applied. For instance, ref. [4] utilizes short-circuited pins in the zero-voltage region of the patch for the TM\({}_{30}\) propagation mode, thus improving the distribution of the dominant TM\({}_{10}\) mode and achieving a fractional bandwidth of 7.5%. In [5], a multi-layer dielectric is proposed to enhance the bandwidth of a patch antenna by modifying the feed structure of the radiator. In [6], the goal is to excite different modes using the same electrical length of the radiator. Under this assumption, the antenna is not limited to operating at a single frequency but can perform at multiple frequencies, thus increasing the bandwidth. Ref. [7] exploits the principle of overlapping multiple resonances, achieved by an array of multiple patches that behave as the main radiator. In [8], a novel combination of fractal geometry and CPW feeding structure is presented to obtain a UWB MIMO antenna, resulting in a highly efficient and reduced-size configuration. In [9], using a two-embedded ring monopole configuration, an mm-wave applications antenna is presented. In this work, the authors obtained a high-efficiency, high-gain, 4-port MIMO antenna, which was very small. On the other hand, in this paper, a compact, easy-to-implement, and built strip-monopole antenna with a significantly widened impedance bandwidth and small size is introduced. The antenna's performance is improved by the inclusion of a T-shaped reactive stub, which stabilizes the impedance and effectively reduces the electrical length.