Abstract: A fast-flared horn antenna for use in a microwave detector comprises an upwardly flared top wall, a downwardly flared bottom wall, and outwardly flared side walls extending between the top wall and the bottom wall. The curvature and the first derivative of the curvature of the flares of the top, bottom and side walls are continuous. Flared and vanishing first and second ridges are formed into the bottom wall and the top wall respectively, the curvature and the first derivative of the curvature of the flares of the first and second ridges are also continuous and sized such that the first and second ridges vanish into the bottom and top walls and the side walls at the same plane. The horn antenna may be sized such that the ridges vanish at the front edge of the aperture of the horn antenna or the ridges may vanish within the horn antenna.