Abstract: A planar antenna is described which employs a thin patch of conductive material supported above and substantially parallel to a closely spaced thin conductive ground surface. Two or more narrow slots are positioned in the ground surface beneath the conductive patch. A microstrip transmission line, placed below the ground surface, excites the slots in series. The length of the microstrip line between the slots, the position of the microstrip line across the slots, and the dimensions of the slots are chosen to excite two orthogonal modes in the conductive patch in phase quadrature. This excitation results in a planar antenna which receives and transmits electromagnetic waves of circular polarization. The antenna may also employ a coplanar waveguide transmission line instead of the aformentioned microstrip transmission line. The coupling apertures then form slot discontinuities in series with the coplanar transmission line, which are positioned under the conductive patch.