Embodiments of present inventions relate to low-profile, tapered cavity broadband antennas. One important aspect of the invention (although not the only) is the incorporation of a tapered lateral sidewalls in the antenna cavity. More particularly, according to various embodiments, a low-profile, tapered cavity antenna may comprise: an aperture defining an opening to a cavity; and an interior space defined by the cavity which is formed of a flat bottom wall defining a ground plane, and a pair of spaced-apart, tapered lateral sidewalls extending away from the flat bottom wall in opposite directions toward the aperture. The tapered shape of the tapered lateral sidewalls are specifically configured to maintain a constant resonance frequency within the cavity. In some embodiments, an isotropic high index medium material is at least partially loaded within the tapered cavity. Also, antenna may include a single or two-input port.