An accurate and numerically efficient model for the electromagnetic coupling between a microstrip line and a microstrip patch via small holes is presented. In the framework of the cavity model both the antenna excitation and the reflection coefficient on the line are computed taking into account all the modes in the cavity. Introducing, in the small aperture coupling, the reaction field as a part of the polarizing field acting on the aperture the theory leads to a physically meaningful equivalent circuit. Both theoretical and measured results are presented, to assess the accuracy of the model.