Abstract: An antenna has a dielectric slab with an inner surface and an outer surface. The inner surface is mounted on a metal backplate. A radiating spiral is located on the outer surface of the dielectric slab and has a pitch of one wavelength at the design frequency. A circular waveguide, for providing RF energy in the TM.sub.01 mode, passes through the metal backplate to terminate at the inner surface of the dielectric slab. A metal cap is located on outer surface of the dielectric slab coaxial with the circular waveguide and at the center of the spiral. The metal cap has a surface is sloped away from the outer dielectric surface and has one or more concentric choke grooves formed on the sloped surface. The grooves are one quarter wavelength deep at the design frequency.