In this communication, we present a low-dispersive leaky-wave antenna consisting of a parallel-plate-waveguide structure loaded by a modulated metasurface and a dense medium. The low-dispersive leaky-wave mode is engineered by a proper design of the metasurface and the chosen permittivity of the dense medium. The dispersion analysis is based on the transverse resonance technique and validated through full-wave simulations. A dense medium with a prism shape and a relative permittivity of 12 is used to radiate a beam pointing at 28.3° in elevation in the Ks-band (27-33 GHz) with only 0.5° variation over a 20% relative bandwidth. The leaky-wave mode supported by the structure is efficiently launched by using a compact quasi-optical system. The full antenna dimensions are 225×175×57 mm3 with an expected average gain of 27.5 dBi and total efficiency greater than 55%.