  The Multimode Equivalent Network (MEN) formulation has been traditionally
employed for the efficient and accurate analysis of waveguide devices. In this
paper, we extend the use of the MEN to the analysis of zero thickness,
multilayer planar circuits in a metallic enclosure. The formulation is
developed for arbitrary shape metallic areas and includes both internal and
external ports in the transverse plane. The BI-RME method is applied to analyze
the arbitrary shaped metallizations and to compute the coupling integrals
needed to solve the corresponding integral equations. On this basis, typical
shielded microstrip circuits of complex geometries, are analyzed in the common
frame of the MEN technique. To validate the theoretical formulation, several
shielded filtering microstrip structures are analyzed, showing good agreement
with respect to both other commercial tools and experimental measurements.
