J. A. Stratton and L. J. Chu were the first scientists who solved an antenna problem with radiation of a ring slot on a conducting sphere [1]. It was almost seventy years ago. There were some inconveniences in that solution. Firstly, the slot width was not taken into consideration so a reactive part of radiation admittance was not defined. Secondly, the mechanism of a slot excitation was not studied so impedance characteristics of the antenna were not described. In this paper the spectral-domain full-wave approach with Green's functions for a spherical layered magnetodielectric structure is suggested for a modified Stretton-Chu's antenna. Radiation patterns and input impedance of a ring slot excited by a coaxial line through a cylindrical resonator are analyzed. An external cover is taken into account.