A cellular-space-division-based method of moments (MoM) algorithm for the analysis of geometries involving imperfectly conducting planar radiators as well as lossy and finiteextent dielectric substrates is presented. Since the technique-via the volume equivalence theorem-replaces the structure under analysis with an equivalent structure composed of thin-wall cells, modeling of the surrounding environment is not required, hence, completely avoiding the need for absorbing boundary conditions. Real (as opposed to perfect) material parameters are incorporated via properly defined surface impedances. Several examples of radiation patterns (including radiation underneath the ground plane of a finite-extent substrate) of planar geometries are presented. The calculated patterns are compared with measured results and are found to be in good agreement. Index Terms-Method of moments, microstrip antennas. I. INTRODUCTION M ODERN planar antenna designs rely heavily on accurate and reliable analysis software. Whereas increasing complexity of printed-circuit radiators demands a high flexibility from the underlying modeling tool, individual numerical techniques are often too restricted in their applications or too CPU time and memory intensive to allow an individual analysis or design to be completed in a timely fashion. Desirable features of suitable numerical models include: 1) the capability of analyzing multilayered circuitry consisting of feed lines, apertures, radiator, and dielectric overlays (e.g., [1]); 2) the incorporation of dielectric and conductor losses (e.g., [2]); 3) the possibility of including finite-size and piecewise homogeneous dielectrics (e.g., [3]); 4) the limitation of the extent of a ground plane (e.g., [4]);