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, and a signal propagating device (62; 82) 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 tank feed through structure including: - a fixed tank connection arranged to be secured to the tank, said fixed tank connection having a through hole with an abutment surface (28a, 28b), - an electrically conducting tank connection adaptor (66; 86) arranged in said through hole in abutment with said abutment surface, - a coupling arrangement (71; 90) arranged in said through hole and resting against said tank connection adaptor, said coupling arrangement providing electromagnetic coupling between said transceiver and said signal propagating device through the feed through structure, characterised in that a fastening member (75; 103) is attached to said fixed tank connection thereby sandwiching said coupling arrangement between said tank connection adaptor and said fastening member, and said signal propagation device is a directional antenna (62; 82), and said tank connection adaptor (66; 86) forms part of a hollow wave guide (60; 80) connecting the transceiver and the directional antenna (62; 82). 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, and a signal propagating device (62; 82) 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 tank feed through structure including: - a fixed tank connection arranged to be secured to the tank, said fixed tank connection having a through hole with an abutment surface (28a, 28b), - an electrically conducting tank connection adaptor (66; 86) arranged in said through hole in abutment with said abutment surface, - a coupling arrangement (71; 90) arranged in said through hole and resting against said tank connection adaptor, said coupling arrangement providing electromagnetic coupling between said transceiver and said signal propagating device through the feed through structure, - a fixed tank connection arranged to be secured to the tank, said fixed tank connection having a through hole with an abutment surface (28a, 28b), - an electrically conducting tank connection adaptor (66; 86) arranged in said through hole in abutment with said abutment surface, - a coupling arrangement (71; 90) arranged in said through hole and resting against said tank connection adaptor, said coupling arrangement providing electromagnetic coupling between said transceiver and said signal propagating device through the feed through structure, characterised in that a fastening member (75; 103) is attached to said fixed tank connection thereby sandwiching said coupling arrangement between said tank connection adaptor and said fastening member, and said signal propagation device is a directional antenna (62; 82), and said tank connection adaptor (66; 86) forms part of a hollow wave guide (60; 80) connecting the transceiver and the directional antenna (62; 82).