A monopole antenna in front of a conducting cylinder is analyzed by the modal-expansion method and scattering matrix method. An infinite perfectly conducting plate is introduced at the top of the monopole antenna to facilitate the modal-expansion analysis. Scattered fields due to the reflecting cylinder are obtained by the scattering matrix method with the aid of cylindrical function's addition theorem. Enforcement of continuity conditions of the tangential field components across the regional surfaces results in the matrix equation for determining the expansion coefficients. Numerical results for the return loss and radiation pattern of a monopole antenna in front of a conducting cylinder are presented and are in good agreement with experimental ones. The effect of the cylindrical reflector's radius and its distance from the monopole on the antenna's radiation performance is also examined.