A planar substrate-superstrate configuration loaded by a graphene sheet is proposed as a novel reconfigurable antenna for terahertz applications. The effects of the introduction of graphene on the dispersion and radiation properties of the fundamental leaky modes supported by the structure are analyzed in detail, with the aim of investigating the possibility of beam scanning at a fixed frequency by varying the graphene chemical potential via an electrostatic bias. After optimizing the graphene position inside the antenna substrate for achieving maximum directivity at broadside, large scanning ranges are obtained in which the normalized attenuation constants of the fundamental TE/TM leaky-mode pair exhibit small values and reduced variation, thus producing a scanning process with an essentially constant narrow beamwidth. In the same ranges, the phase constants of the TE/TM-mode pair are shown to be …