A thick monopole antenna fed by a circumferential slot cut on the outer conductor of a coaxial cable is studied with rigorous field theory in this paper. In our analysis we consider not only the higher-order modes inside the coaxial cable but also the nonuniform distribution of the electric field in the slot. The coupled integrodifferential equations are formulated for the evaluation of the current distribution on the antenna surface as well as the electric field distribution in the slot. The restriction of the slot width and the radii of the antenna are removed from the analysis, The effect of the slot width on the electric field distribution in the slot, the current distribution on the outer conductive surface of the antenna, and the input admittance viewed in the coaxial cable are presented and discussed. The calculated input admittance is in good agreement with the experimental result.