An insulated linear antenna consists of a metallic, cylindrical conductor covered by a concentric sheath of dielectric with relative permittivity /spl epsiv//sub ri/. Insulated antennas are almost always used in an ambient medium, such as soil, seawater, or biological tissue, whose electrical properties are quite different from those of the insulation; that is, either the relative permittivity /spl epsiv//sub re/ or the conductivity /spl sigma//sub e/, of the external medium is much greater than that of the insulation. The purpose of this paper is to establish the range of validity for the transmission line theory for the insulated monopole antenna to produce accurate results. This is accomplished by comparing results from the transmission line theory with accurate calculations made with the finite-difference time-domain (FDTD) method.