In this letter, we present the design and the experimental realization of an innovative self-filtering low-noise horn antenna. The proposed radiator consists of the following: a regular WR-62 waveguide connected to a horn antenna, a metallic screen with a vertical slit placed at the section connecting the waveguide and the horn, and a dielectric slab, with metallic omega shapes printed on both faces, placed across the slit. The goal of this design is to reduce the bandwidth of operation of the regular WR-62 horn in order to self-filter the noise captured within the antenna operation band. In the receiving mode, the metallic omega shapes placed across the slit allow transmission only in a narrow frequency band centered around the resonant frequency of the omegas. In such a frequency band, the radiating performances of the proposed antenna are comparable to the ones of a regular WR-62 horn radiator. A proper set of numerical simulations and measurements confirm the effectiveness of the proposed design, which can be successfully used in receiving satellite communication systems.