An approximation for the diffraction of an edge wave by the vertex and edges of a conducting trihedron is developed. The edge wave is produced by an electric dipole source located in the vicinity of one of the edges and sufficiently far from the tip of the vertex. The radiation integral of the currents that would be induced over an infinite wedge, which is then truncated, is evaluated asymptotically yielding an endpoint effect. The effect of a fringe current component, excited by the terminating edges, is introduced subsequently. The approximation shows good agreement with method of moments computations and measured data for several flat plate structures.