How do I optimize the gain of a Fabry-Perot cavity antenna?

A Fabry-Perot cavity antenna achieves high gain (high directivity) by placing a partially reflective surface (PRS) parallel to a ground plane at a specific height, creating a resonant cavity that produces a narrow, highly directive beam from a simple feed (such as a slot or patch antenna). The gain optimization involves: selecting the cavity height (h = lambda/2 at the operating frequency for maximum gain; the partially reflective surface and ground plane form a parallel plate resonator, and at h = lambda/2, the multiple reflections within the cavity add constructively in the broadside direction), designing the PRS reflectivity (the PRS is a periodic structure, such as a metallic mesh, patch array, or dielectric slab, with reflectivity |rho| typically 0.7-0.95; higher reflectivity increases gain but narrows bandwidth and increases Q), and optimizing the PRS unit cell (the PRS can be implemented as: a metallic grid (high-pass PRS), a patch array (low-pass PRS), or a multi-layer PRS (combining both for wider bandwidth)). The maximum achievable gain is: G_max approximately pi^2 / (1 - |rho|^2) for a large enough cavity, practically limited by the cavity area (A approximately lambda^2 x G / (4 pi)) and the PRS reflectivity. Typical gains: 15-20 dBi from a single-feed FPC antenna with a cavity area of approximately 3 lambda x 3 lambda. The bandwidth is inversely proportional to the gain: higher reflectivity (higher gain) means narrower bandwidth, typically 2-5% for gains of 15-20 dBi.