Radar antenna (1) for the transmission and reception of radar waves (4) for measuring the level of a surface of a liquid enclosed in a tank, wherein the radar antenna (1) is arranged above the surface (3a) of the liquid and substantially directed downwards towards said surface and said level determined by evaluation of the time lapsed between the transmission and reception of a signal included in said radar waves and wherein said radar antenna (1) has a front surface (1a) for transmission and reception of said radar waves, wherein said surface (1a) of the antenna, wherein said sheet has a sheet surface (1b) between said antenna front surface (1a) and the surface (3a) of the liquid and said sheet surface (1b) being arranged in a predetermined angle α being greater than 0° in relation to the horizontal plane H, whereby a fluid occurring on the surface (1b) of the sheet is drained off by means of gravity.