A dual-frequency patch antenna includes a ground plane, an inside radiator, and an outside radiator. The inside radiator is configured as a region with a periphery, along which is a series of first protrusions separated by first grooves. The outside radiator is configured as a ring with an outer periphery and an inner periphery, along which is a series of second protrusions separated by second grooves. A set of conducting elements electrically connect the series of second protrusions with the ground plane. The inside radiator and the outside radiator can be fabricated on a dielectric substrate separated from the ground plane by a dielectric solid or air. The inside radiator and the outside radiator can be disposed on the same surface or on different surfaces of the dielectric substrate, with specific geometries of the first protrusions and first grooves relative to the second protrusions and second grooves.