This paper presents a spectral-domain analysis of a 2 x 2 microstrip antenna array fed by a microstrip line. The Galerkin Method of Moments is employed to gain insight into the characterization and performance of such a patch array antenna system, whereas the mutual coupling is taken into account in the analysis. The VSWR and the radiation pattern of the array are calculated and compared with the data of measurements made in a frequency range from 9 to 11 GHz. As a further extension of this analysis of the 2 x 2 array, the features of a 4 x 4 microstrip patch array are also investigated in this paper and some interesting results are obtained. As shown in the comparison, reasonably good agreement between theoretical analysis and experimental data is achieved.