This paper presents a full-wave method, based on the method of moments (MoM), to calculate the \(\bar{\varvec{S}}\) matrix from a two-dimensional complex sample in millimeter and submillimeter W and J bands. From the surface currents obtained by inverting the impedance matrix and from the Huygens principle, the reflection and transmission coefficients are computed. This allows us to obtain the four elements of the \(\bar{\varvec{S}}\) matrix. Firstly, the method is validated from canonical samples (a dielectric slab and a stack of two dielectric slabs) by applying the well-known Fresnel coefficients. Secondly, for the W (75 to 110 GHz) and J (220 to 330 GHz) bands, a PVC slab covered by water drops is considered, for which the \(\bar{\varvec{S}}\) matrix is compared with measurements made in quasi-optical free space. A satisfactory agreement is obtained between the measurements and the model.
