In this paper, a numerical analysis of shielding properties of a protective metal enclosure has been performed by simulating a receiving antenna placed inside. Here, the main focus is put on the monopole antenna type. This antenna type is used in an experimental set-up to measure the shielding effectiveness (SE) of the enclosure as often as some other antennas such as dipole. Numerical model, based on the Transmission-Line Matrix (TLM) method, is applied to numerically investigate an impact this antenna might have on a detected level of electromagnetic (EM) field when it is used in the SE measurements. As in experiments, the estimation of SE of an enclosure in the numerical model is performed based on the current induced on the antenna, directly provided by a compact TLM wire model. The model accuracy is verified through a comparison with the experimental results from the literature. The analysis includes the monopole antenna with different radii and lengths, in order to consider changes in the resonant frequencies and the SE level of enclosure. Finally, the enclosure with either the monopole or the dipole is simulated in order to compare their influence on EM field distribution inside the enclosure.