The characterization of metal–dielectric isotropic metasurfaces is investigated in this work to efficiently design a Fabry–Perot cavity leaky-wave antenna in the THz range. In particular, the theoretical response of a fishnet-like metasurface—realized by patterning a 200-nm aluminum film over a 500-${\mu}$m-thick silica substrate through standard photolithographic techniques—is experimentally validated by means of THz time-domain spectroscopy measurements in reflection mode. The obtained metasurface representation has then been exploited for designing a 2-D leaky-wave antenna working at 300 GHz, which is originally based on the radiation of the second-order, TE-TM leaky-mode pair in the cavity.