An antenna (e.g., circularly polarized (CP) planar aperture antenna) is presented herein. An antenna can include an opening cavity and a patch section within a perimeter of the opening cavity. The patch section can have crossed patch in a windmill shape. The opening cavity can have a windmill shape. The patch section can be fed differentially via grounded co-planar waveguide (GCPW). The antenna can be formed in a single layer substrate and can be 180° rotationally symmetric about its center. A height of the opening cavity can be quarter-wavelength at operating frequency and a diameter of the opening cavity can be larger than one wavelength at operating frequency.