A radar level gauge comprising a directional antenna, a hollow wave guide connected to the antenna, a tank connection having a through hole with an annular abutment, a dielectric filling member arranged in the tank connection, the dielectric filling member comprising an annular collar portion resting against the annular abutment, a clamping member inserted into the tank connection, a fastening member attached to the fixed tank connection to secure the clamping member between the fastening member and the annular abutment, and a spring member arranged between the fastening member and the clamping member. The spring member presses the clamping member towards the annular abutment, to apply a controlled pressure on the annular collar portion, allowing the annular collar portion to serve as a pressure tight sealing between a tank interior and a tank exterior.