A radar level gauge for determining a filling level of a product in a tank using electromagnetic measuring signals, the radar level gauge comprising: a transceiver comprising a transmitter for transmitting an electromagnetic transmit signal and a receiver for receiving an electromagnetic echo signal, processing circuitry connected to the transceiver for determining the filling level based on the electromagnetic echo signal, a transmission line probe connected to the transceiver and arranged to extend into the product of the tank, the transmission line probe being adapted to direct the electromagnetic transmit signal towards a surface of the product and to return a reflection from the surface as the electromagnetic echo signal; and a tank feed through structure comprising: a tank fixing connection arranged to secure the feed through structure to the tank, the tank fixing connection comprising a through hole with an abutment surface, a dielectric sleeve comprising an outer envelope surface arranged in the through hole of the tank fixing connection and in abutment with the abutment surface, and an inner surface defining a trough hole; and an elongated and axially extending transmission member arranged in the through hole of the dielectric sleeve, the transmission member comprising a first axial end portion connected to the transceiver and a second axial end portion connected to the transmission line probe, wherein an envelope surface of the transmission member comprises at least one radially extending protrusion between the first and second axial end portions, wherein the inner surface of the dielectric sleeve is in abutment with, and encloses, the radially extending protrusion axially above, axially below and on the radially extending protrusion such as to conform with the shape of the at least one radially extending protrusion characterized in that the inner surface of the dielectric sleeve is in abutment with, and encloses, the radially extending protrusion, the dielectric sleeve and the transmission member being sintered to each other. a transceiver comprising a transmitter for transmitting an electromagnetic transmit signal and a receiver for receiving an electromagnetic echo signal, processing circuitry connected to the transceiver for determining the filling level based on the electromagnetic echo signal, a transmission line probe connected to the transceiver and arranged to extend into the product of the tank, the transmission line probe being adapted to direct the electromagnetic transmit signal towards a surface of the product and to return a reflection from the surface as the electromagnetic echo signal; and a tank feed through structure comprising: a tank fixing connection arranged to secure the feed through structure to the tank, the tank fixing connection comprising a through hole with an abutment surface, a dielectric sleeve comprising an outer envelope surface arranged in the through hole of the tank fixing connection and in abutment with the abutment surface, and an inner surface defining a trough hole; and an elongated and axially extending transmission member arranged in the through hole of the dielectric sleeve, the transmission member comprising a first axial end portion connected to the transceiver and a second axial end portion connected to the transmission line probe, wherein an envelope surface of the transmission member comprises at least one radially extending protrusion between the first and second axial end portions, wherein the inner surface of the dielectric sleeve is in abutment with, and encloses, the radially extending protrusion axially above, axially below and on the radially extending protrusion such as to conform with the shape of the at least one radially extending protrusion a tank fixing connection arranged to secure the feed through structure to the tank, the tank fixing connection comprising a through hole with an abutment surface, a dielectric sleeve comprising an outer envelope surface arranged in the through hole of the tank fixing connection and in abutment with the abutment surface, and an inner surface defining a trough hole; and an elongated and axially extending transmission member arranged in the through hole of the dielectric sleeve, the transmission member comprising a first axial end portion connected to the transceiver and a second axial end portion connected to the transmission line probe, wherein an envelope surface of the transmission member comprises at least one radially extending protrusion between the first and second axial end portions, wherein the inner surface of the dielectric sleeve is in abutment with, and encloses, the radially extending protrusion axially above, axially below and on the radially extending protrusion such as to conform with the shape of the at least one radially extending protrusion