A monopole is mounted on a perfectly conducting (PEC) planar screen of infinite extent in which an annular s1ot of finite depth is cut. A pair of coupled integral equations are derived for this configuration. The electric current on the monopole and the electric field (or equivalent magnetic current) at the interface between the slot and the half space are determined as the solution of these integral equations. The input impedance of the monopole and the far field pattern are computed from these electric current and equivalent magnetic current.