A compact wideband RF antenna for incorporating into a planar substrate, such as a PCB, having at least one cavity with a radiating slot, and at least one transmission line resonator disposed within a cavity and coupled thereto. Additional embodiments provide stacked slot-coupled cavities and multiple coupled transmission-line resonators placed within a cavity. Applications to ultra-wideband systems and to millimeter-wave systems, as well as to dual and circular polarization antennas are disclosed.