A radar level gauge using electromagnetic waves for determining a filling level of a product in a tank, said radar level gauge comprising: a transceiver for transmitting an electromagnetic transmit signal and receiving an electromagnetic echo signal; processing circuitry connected to said transceiver for determining said filling level based on said electromagnetic echo signal; a directional antenna (11; 111) for directing said electromagnetic transmit signal towards a surface of said product and to return a reflection from said surface as said electromagnetic echo signal; a hollow wave guide (20;120) connecting said transceiver with said antenna, a tank connection (23; 123) arranged to be sealingly secured to the tank, said fixed tank connection having a through hole (25; 125) with an annular abutment (52; 152); a dielectric filling member (31; 131) arranged in the tank connection and configured to prevent entry of tank content into the antenna and/or hollow wave guide, the dielectric filling member comprising an annular collar portion (33; 133) resting against said annular abutment; a clamping member (38; 138, 138a, 138b) inserted into the tank connection, such that the annular collar portion is sandwiched between the annular abutment and the clamping member; and a fastening member (43; 143) attached to said fixed tank connection thereby securing said clamping member between said fastening member and said annular abutment; characterized by a spring member (51; 151) arranged between said fastening member and said clamping member, and configured to press said clamping member towards the annular abutment, so as to apply a controlled pressure on said annular collar portion, allowing said annular collar portion to serve as a pressure tight sealing between a tank interior and a tank exterior. a transceiver for transmitting an electromagnetic transmit signal and receiving an electromagnetic echo signal; processing circuitry connected to said transceiver for determining said filling level based on said electromagnetic echo signal; a directional antenna (11; 111) for directing said electromagnetic transmit signal towards a surface of said product and to return a reflection from said surface as said electromagnetic echo signal; a hollow wave guide (20;120) connecting said transceiver with said antenna, a tank connection (23; 123) arranged to be sealingly secured to the tank, said fixed tank connection having a through hole (25; 125) with an annular abutment (52; 152); a dielectric filling member (31; 131) arranged in the tank connection and configured to prevent entry of tank content into the antenna and/or hollow wave guide, the dielectric filling member comprising an annular collar portion (33; 133) resting against said annular abutment; a clamping member (38; 138, 138a, 138b) inserted into the tank connection, such that the annular collar portion is sandwiched between the annular abutment and the clamping member; and a fastening member (43; 143) attached to said fixed tank connection thereby securing said clamping member between said fastening member and said annular abutment; characterized by a spring member (51; 151) arranged between said fastening member and said clamping member, and configured to press said clamping member towards the annular abutment, so as to apply a controlled pressure on said annular collar portion, allowing said annular collar portion to serve as a pressure tight sealing between a tank interior and a tank exterior.