An automated radiophysical complex operating in the millimeter wavelength range is presented. We have measured the spatial amplitude and phase distributions of the electromagnetic field in its two orthogonal components for the case of diffraction of electromagnetic waves incident upon an object at near-grazing angles. Thw wave diffraction at the aperture and circular slot in the metal shield is investigated. The directivity patterns and transmission coefficient as functions of the relative aperture diameter and circular slot are given. The extremum in the dependence of the maximum angle on the width of the circular slot is found.