The dielectric overlay effects on the impedance properties of a radiating slot in the ground plane of a microstrip line are studied by using the reciprocity theorem and a moment-method calculation. The Green's functions for the dielectric-covered microstrip line-fed radiating slot are given, and the expression for the slot impedance is shown. Numerical convergence of the moment-method calculation using, respectively, piece-wise sinusoidal basis functions and rooftop basis functions is discussed. Numerical results for the slot impedance as a function of overlay thickness and permittivity are presented and discussed. (C) 1994 John Wiley & Sons, Inc.