The efficiency of the moment method when the expansion functions are defined in an infinite domain is investigated. It is shown that an efficient solution is obtained when the expansion functions exactly obey the known physical behavior of the fields. The age-old problem of a patch microstrip disk antenna excited by a probe at its center is used as a toy model. This antenna is solved by the moment method for which an electric field component is expanded outside the disk. Excellent agreement between the conventional moment solution based on expanding the surface current density on the disk and the present solution is obtained.