Embodiments of the present invention relate to low-profile broadband antennas having an anisotropic traverse resonance condition. One important aspect of the invention is the incorporation of an anisotropic high index medium material, at least partially loaded within the cavity, which is configured to maintain a constant resonance frequency of the antenna. A low-profile cavity antenna may comprise: an aperture defining an opening to a cavity; 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, lateral sidewalls extending away from the flat bottom wall in opposite directions toward the aperture; and an anisotropic high index medium material, at least partially loaded within the cavity, configured to maintain a constant resonance frequency of the antenna. The lateral sidewalls may extend from opposing sides of the flat bottom wall perpendicularly or with an outwardly taper.