A radar level gauge system, comprising a transceiver; an antenna for radiating an electromagnetic transmit signal from the transceiver towards a surface of the product and for returning an electromagnetic reflection signal back towards the transceiver; a feed-through connecting the transceiver and the antenna; and processing circuitry coupled to the transceiver for determining the filling level based on a relation between the transmit signal and the reflection signal, wherein the feed-through comprises a waveguide arranged between the transceiver and the antenna to receive the transmit signal from the transceiver and guide the transmit signal towards the antenna in a guiding direction, the waveguide comprising an elongated plug arranged in a hollow conductor extending in the guiding direction, wherein the plug comprises a non-conductive sleeve member and a metallic plug member, the non-conductive sleeve member being sealingly joined to a portion of the hollow conductor and to the metallic plug member.