The paper presents the results of a comprehensive study of the characteristics of a microstrip MIMO antenna system intended for application in fifth-generation communication equipment operating within the n79 frequency band (4.4–5.0 GHz). The purpose of the research was to develop a compact 1×2 antenna array on a single dielectric substrate and to evaluate its electrodynamic properties, taking into account factors capable of influencing matching and radiation parameters under real operating conditions. The proposed design incorporates two radiating elements, four excitation ports, and a modified ground plane, which provides enhanced functional stability and improved operational characteristics. Numerical modelling demonstrated that the developed antenna system ensures stable standing-wave ratio values, high antenna gain, and consistent radiation patterns across the entire operating frequency band. Particular attention was devoted to assessing the influence of external environmental effects, especially the formation of a thin water film on the radiating surface. The study revealed that the presence of moisture results in an SWR increase of more than 0.5, a decrease in antenna efficiency, and degradation of radiation characteristics at higher frequencies, highlighting the necessity of employing protective structural solutions when operating under humidity-prone conditions. Additionally, the feasibility of replacing copper with an ideal conductor in electromagnetic simulations was examined. The difference in SWR between the realistic and idealized models did not exceed 0.006, confirming that such a simplification may be applied during early design stages without significant loss of accuracy. An experimental prototype of the antenna was fabricated and tested under laboratory conditions. The obtained measurements demonstrated a high degree of agreement with the modelling results, validating the proposed mathematical model and confirming the practical applicability of the developed MIMO antenna system in next-generation communication infrastructures
