A one-dimensional zonal reflector is a conducting surface which is uniform in one direction and has a zonal profile in the other. Similar to a cylindrical reflector, it converts a cylindrical wave from a line-source into an outgoing plane wave. In this paper, the radiation performance of such reflectors with TM-wave illumination is analyzed by the method of moments (MoM). The influence of corner diffraction and zoning on the sidelobe level is investigated. Three types of configurations are considered, which include the parabolic zonal reflector and two stepwise zonal reflectors of different configurations. It is found that corner diffraction makes a significant contribution to the sidelobe level, but zoning intrinsically raises the far-out sidelobes. Numerical results show that the near-in sidelobes of the stepwise zonal reflector can be significantly decreased by adjusting the geometrical configuration.