A flat metamaterials (MTMs) structure with low refractive index is proposed as a cover in a high-gain horn antenna configuration. The MTMs is composed of two-layer metallic grids and slices of foam. For the characterization of the MTMs, the low refraction property is studied and the effective refractive index n ≈ 0.08 (12.0 GHz) is retrieved. The antenna gain and the radiation pattern are calculated. Because of the electromagnetic wave congregating effect of the MTMs, the gain of the horn with MTMs greatly increases (about 4 dB from 11.8 GHz–12.8 GHz) when compared with the conventional type. We conducted the experiments to verify the simulation results. The idea has a good potential application to the feed design of parabolas.