The present invention relate to a
horn antenna at a
radar level gauge for determination of a level of a surface of a medium stored in a tank and a method of making a
horn antenna which is mechanically constituted such that the
horn antenna will withstand the environment of the tank interior. The antenna comprises a first conductive housing (7) having a wave guide port (8), which is connectable to a wave guide, a first aperture (10), which is separated from the wave guide port (8) and a
fist divergent section between the wave guide port (8) and the first aperture (10). A
dielectric body (11) is arranged inside and essentially filling the first conductive housing (7), which body (11) displays a free end (12) in the area of the first aperture (10). The horn antenna is characterized in that around the first aperture (10) is arranged a locking device (13), which is arranged to fix the
dielectric body (11) in the first housing (7) and that in the area of the locking device (13) is arranged at least one flexible sealing element (16) between the
dielectric body (11) and the first cover (7).