This letter presents a numerical study of the best performance achievable using a spherical reflector with a point feed and without any corrective element. We evaluate in detail the aperture efficiency that can be obtained with a spherical reflector with linearly polarized Gaussian beam illumination. A 100 GHz, 30 cm diameter spherical reflector from a parent sphere of 30 cm radius has been built and measured. The maximum directivity of 46.1 dB agrees closely with the predictions of electromagnetic modeling, as does the increase in the directivity by more than 3 dB resulting from moving the feed by 0.7 cm closer to the reflective surface relative to the paraxial focal point.@en