A radar level gauge system for determining a filling level of a product in a tank, comprising: a transceiver for generating, transmitting and receiving electromagnetic signals; an antenna for radiating an electromagnetic transmit signal from the transceiver towards a surface of the product and for returning an electromagnetic reflection signal resulting from reflection of the electromagnetic transmit signal at the surface back towards the transceiver; a hollow waveguide for guiding the transmit signal in a signal propagation direction from a first end of the hollow waveguide facing the transceiver towards a second end of the hollow waveguide facing the antenna; a housing enclosing at least the transceiver and the hollow waveguide, the housing including a heat-dissipating structure (39; 41) arranged at a first distance, in the signal propagation direction, from the second end of the hollow waveguide; a first thermal connection between the hollow waveguide and the heat-dissipating structure; and a second thermal connection between the hollow waveguide and the housing arranged at a second distance, in the signal propagation direction, from the second end of the hollow waveguide, the second distance being shorter than the first distance, characterised in that the first thermal connection and the second thermal connection are dimensioned and arranged in such a way that a thermal resistance of a first heat conduction path from the second end of the hollow waveguide through the first thermal connection to the heat-dissipating structure, is lower than a thermal resistance of a second heat conduction path from the second end of the hollow waveguide through the second thermal connection to the housing. a transceiver for generating, transmitting and receiving electromagnetic signals; an antenna for radiating an electromagnetic transmit signal from the transceiver towards a surface of the product and for returning an electromagnetic reflection signal resulting from reflection of the electromagnetic transmit signal at the surface back towards the transceiver; a hollow waveguide for guiding the transmit signal in a signal propagation direction from a first end of the hollow waveguide facing the transceiver towards a second end of the hollow waveguide facing the antenna; a housing enclosing at least the transceiver and the hollow waveguide, the housing including a heat-dissipating structure (39; 41) arranged at a first distance, in the signal propagation direction, from the second end of the hollow waveguide; a first thermal connection between the hollow waveguide and the heat-dissipating structure; and a second thermal connection between the hollow waveguide and the housing arranged at a second distance, in the signal propagation direction, from the second end of the hollow waveguide, the second distance being shorter than the first distance, characterised in that the first thermal connection and the second thermal connection are dimensioned and arranged in such a way that a thermal resistance of a first heat conduction path from the second end of the hollow waveguide through the first thermal connection to the heat-dissipating structure, is lower than a thermal resistance of a second heat conduction path from the second end of the hollow waveguide through the second thermal connection to the housing.