PURPOSE: To converge beam width and to prevent snow or water from being attached by providing a miniature helical antenna composed of the small number of winding in a bottomed cylindrical horn reflector with the aperture surface spread from the bottom, spreading an opening surface from the bottom surface and providing a dielectric board on the opening surface.

CONSTITUTION: A miniature helical antenna 13 is provided with the small number almost twice of winding while insulating and separating the bottom surface in the opening of a horn reflector 11, which is composed of a metallic body with the bottomed cylindrical shape, in the central part of a reflection board 12. A dielectric board 15 to block the opening surface of the reflector 11 is provided so as to be freely attached and detached to a projecting part 11a formed on the outer periphery of the opening surface of he reflector 11. Thus, the spread of the beam width caused by the antenna 13 can be converged to the suitable beam width by the reflector 11 and the snow or water can be prevented from being adhered. Accordingly, even when reducing the number of helical winding, the antenna beam width can be prevented from being spread and satisfactory weather proof can be obtained.