In this report, a method is developed for the quantitative evaluation of the effects of large irregularities or obstacles on the propagation of a radio wave. The fundamental tool used is Keller's "geometrical theory of diffraction." In contrast to previous methods for treating this problem, the one proposed here is not limited by 1) the type of terrain, 2) the polarization of the transmitting antenna, 3) the radiation pattern of the antenna, and 4) the frequency of the antenna (provided ka>̲ 5.0, where a is the characteristic dimension of the diffracting obstacle). A numerical example showing the antenna pattern behind a hemispherical hill illustrates the application of the theory.