An approximate model for the patch element, based on cavity model expansion, is adopted. The model has been improved by taking into account the effect of the substrate by constructing a dyadic Green's function for the grounded dielectric substrate utilizing the rectangular vector wave functions (RVWF). The reaction concept has been used to evaluate the mutual impedance between the patches. The integrations over the source region to evaluate the H-fields as well as those in the mutual impedance formula have been performed analytically. The remaining integrations which are in spectral domain have been evaluated numerically by using the fast Fourier transform (FFT). It was found that the method can be used to evaluate the mutual impedance between microstrip patch antennas with great accuracy for typical substrate thicknesses in L- and C-bands. The agreement between the calculated results and measurements have been found to be excellent. It is expected that the method will also work reasonably well in higher frequency bands.
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