An antenna system is described which is comprised of an artificial magnetic conductor (AMC), an antenna element, and a feed network comprised of shielded feedlines whose outer conductor, or shield, is routed through the substrate of the AMC. The feedline outer conductor is connected to both the substantially continuous conductive surface and the array of capacitive patches forming the AMC. The shielded feedline suppresses the excitation of undesired TM modes within the AMC substrate, results in a stable return loss over a frequency range associated with the AMC's high surface impedance and surface wave bandgap.