Fabry-Perot cavity antennas (FPCAs) are constituted by a grounded dielectric slab with a partially reflecting surface (PRS) on top, commonly implemented by an isotropic metallic thin sheet acting as a metasurface. These devices can be designed to radiate directive beams at broadside through a low-profile and compact structure at the expense of a typically narrowband behavior. The latter issue has recently been mitigated by placing a dielectric superstrate over the metasurface, generating a so-called thick PRS (see Fig. 1). This additional layer can suitably be designed to obtain a positive slope on the phase of the reflection coefficient, i.e., the thick PRS can show a non-Foster behavior. It is therefore possible to design the PRS in order to fulfill the cavity resonance condition to have a broadside beam over a relatively large frequency range.