The field of a quarter‐wave‐length antenna above a circular conducting disk of zero thickness—the groundplane—is calculated theoretically by use of the wave functions of the oblate spheroid. Assuming a sinusoidal distribution of current on the antenna, we compute the currents on the groundplane, the radiation resistance and the radiation pattern of the system for various values of the radiusa. With these results the distortion of antenna radiation by finite groundplanes can be studied. We find good agreement with recent experimental data.