An analytically simple and numerically efficient technique for calculating the pattern distortion of aperture antennas radiating in the presence of conducting platforms located in the near or far field of the antenna is presented. The technique presented, based on uniform geometrical theory of diffraction (UGTD), is also applicable for large aperture antennas (aperture area

). An excellent agreement between the calculated and measured results obtained for a typical aperture antenna mounted on a conducting platform confirms the validity of the analytical technique developed.