An antenna horn includes a waveguide having an open end and an end allowing access to transmitted signals, the widest opposite walls constituting a first pair of walls, two first ridges inside the waveguide, in the middle and over the whole length of the walls of the first pair of walls, a flat central wall connecting the walls of the second pair of walls at their midpoints at the level of the accesses, stopping in the direction of the open end so as to polarize signals transmitted by the two accesses according to orthogonal circular polarizations, and forming two ridges in the middle of the walls of the second pair of walls from the side of the open end, and with an antenna, an item of radio communication equipment and a method using the horn.