Abstract: A horn that radiates a radioelectric wave coming from an input waveguide, comprises a grating placed over the aperture of the horn. The waveguide comprises a horn-shaped segment, an entrance, an aperture, and a grating placed next to the aperture. It makes it possible for at least one linearly polarised electromagnetic wave to propagate between the entrance and the aperture along a first axis. The grating comprises a frame and a set of plates extending longitudinally and continuously from a first short side of the frame to a second short side of the frame, so as to form a linear polarising filter for any electromagnetic wave the electric field of which is not polarised along a second axis orthogonal to the first axis. The grating of the waveguide comprises corrugations to reinforce the filtering of the electromagnetic wave the electric field of which is not polarised along the second axis.