A low profile, wideband resonant cavity antenna (RCA) is presented. It uses a simple and planar wideband feed antenna. The use of this planar, printed feed antenna reduces the overall height of the RCA. The RCA has a single-layer superstrate, which has radially non-uniform permittivity in the transverse plane. Excellent wideband matching was obtained for the RCA with a -10dB return loss bandwidth ranging from 20.81 to 25.32 GHz. Numerical results predict a peak boresight gain of 16.97 dBi. This RCA is well-suited for scalable RCA configurations such as sparse arrays or switched-polarization arrays with printed feed networks.