A guided wave radar level gauge system, for determination of a filling level of a product contained in a tank, said guided wave radar level gauge system comprising: a transceiver (10; 11) for generating, transmitting and receiving electromagnetic signals in a first frequency range and in a second frequency range different from said first frequency range; a transmission line probe, having one probe conductor only, connected to said transceiver (10; 11), and arranged and configured to guide transmitted electromagnetic signals from said transceiver through a surrounding medium, surrounding the transmission line probe, towards said product inside the tank, and to return reflected electromagnetic signals resulting from reflection of said transmitted electromagnetic signals by a surface of said product back towards said transceiver, the transmission line probe being configured to guide electromagnetic signals in said first frequency range with a first coupling to the surrounding medium resulting in a first propagation velocity exhibiting a first dependence on a dielectric constant of the surrounding medium, and guide electromagnetic signals in said second frequency range with a second coupling to the surrounding medium resulting in a second propagation velocity exhibiting a second dependence, different from the first dependence, on said dielectric constant of the surrounding medium; and processing circuitry connected to the transceiver (10; 11) for determining the filling level based on a first reflected electromagnetic signal in said first frequency range, a second reflected electromagnetic signal in said second frequency range, and a known relation between said first dependence and said second dependence on said dielectric constant of the surrounding medium. a transceiver (10; 11) for generating, transmitting and receiving electromagnetic signals in a first frequency range and in a second frequency range different from said first frequency range; a transmission line probe, having one probe conductor only, connected to said transceiver (10; 11), and arranged and configured to guide transmitted electromagnetic signals from said transceiver through a surrounding medium, surrounding the transmission line probe, towards said product inside the tank, and to return reflected electromagnetic signals resulting from reflection of said transmitted electromagnetic signals by a surface of said product back towards said transceiver, the transmission line probe being configured to guide electromagnetic signals in said first frequency range with a first coupling to the surrounding medium resulting in a first propagation velocity exhibiting a first dependence on a dielectric constant of the surrounding medium, and guide electromagnetic signals in said second frequency range with a second coupling to the surrounding medium resulting in a second propagation velocity exhibiting a second dependence, different from the first dependence, on said dielectric constant of the surrounding medium; and processing circuitry connected to the transceiver (10; 11) for determining the filling level based on a first reflected electromagnetic signal in said first frequency range, a second reflected electromagnetic signal in said second frequency range, and a known relation between said first dependence and said second dependence on said dielectric constant of the surrounding medium.