In this paper, the radiation properties of an annular slot antenna of infinitesimal aperture and radius a, cut in an infinite ground plane and covered by a dielectric cylinder of finite height, are investigated analytically. The excitation is assumed to be a circumferentially constant voltage at the slot opposite edges. The unknown electromagnetic field is expressed using the mixed mode spectrum of a radial dielectric waveguide. Then, by enforcing the satisfaction of the appropriate boundary conditions, an integral equation is derived in terms of the unknown electric field distribution Ez. This equation is solved by employing Galerkin's technique and the electromagnetic field distributions are computed in all the regions of the 3-D space. Finally, based on these results, the input impedance and the far field pattern of the antenna are obtained. Numerical results are presented for several cases.