Abstract: A microwave antenna (10) is excited by two independently driven feed lines (30 and 32) so that the plane of polarization of the resultant microwave radiation depends on the relative amplitudes of the signals on the two feed lines. The feed lines are disposed between two parallel ground planes (12 and 14), in one of which is etched an aperture (18). The periphery of the aperture forms ridges (20, 22, 24, and 26) in registration with the feed lines (30 and 32). Conductive eyelets (27) extend between the ground-plane conductors (12 and 14) to act as shorting elements that surround the aperture to form a cavity defined by the shorting elements and the ground-plane conductors. The resultant antenna, which may, for instance, have a thickness of only one-tenth of a wavelength, achieves a 2:1 VSWR bandwidth on the order of 30%.