The design of a low-profile narrowband antenna that is tunable over a wide frequency range is presented. A spline-based Ultra-Wideband (UWB) antenna is synthesized by means of a time-domain-based PSO and is mounted on top of a reconfigurable Artificial Magnetic Conductor (AMC) obtained by replicating a square unit cell in a 15×15 periodic lattice. The AMC is composed by a planar array of periodic metallic strips printed on top of a PEC-backed dielectric substrate, connected by tunable capacitors (varactors), and has been designed to work within the 1-4 [GHz] bandwidth. The reflection coefficient of the resulting narrowband antenna system can be easily controlled inside a wide frequency range by tuning the capacitance value C of the varactors inside the AMC groundplane.