A radar level gauge system, for determining a filling level of a product in a tank, said radar level gauge system comprising: a transceiver arrangement for generating, transmitting and receiving electromagnetic signals; a first transmission line probe arranged and configured to guide an electromagnetic first transmit signal (S T1) from said transceiver arrangement through a surrounding medium towards said product inside the tank, and to return an electromagnetic first surface echo signal (S R1) resulting from reflection of said first transmit signal by a surface of said product back towards said transceiver arrangement, the first transmission line probe comprising a first transmission line probe conductor and a first dielectric enclosing structure at least partly enclosing said first transmission line probe conductor along an entire length of the transmission line probe arranged inside said tank, so that said first transmission line probe provides a first coupling efficiency between said first transmit signal and the surrounding medium, to thereby guide said first transmit signal with a first propagation velocity exhibiting a first dependence on a dielectric constant of the surrounding medium; a second transmission line probe arranged and configured to guide an electromagnetic second transmit signal (S T2) from said transceiver arrangement through the surrounding medium towards said product inside the tank, and to return an electromagnetic second surface echo signal (S R2) resulting from reflection of said second transmit signal by the surface of said product back towards said transceiver arrangement, the second transmission line probe being configured to provide a second coupling efficiency between said second transmit signal and the surrounding medium, to thereby guide said second transmit signal with a second propagation velocity exhibiting a second dependence, different from the first dependence, on the dielectric constant of the surrounding medium; and processing circuitry connected to the transceiver arrangement for determining the filling level based on said first surface echo signal, said second surface echo signal, and a known relation between said first dependence and said second dependence on the dielectric constant of the surrounding medium. a transceiver arrangement for generating, transmitting and receiving electromagnetic signals; a first transmission line probe arranged and configured to guide an electromagnetic first transmit signal (S T1) from said transceiver arrangement through a surrounding medium towards said product inside the tank, and to return an electromagnetic first surface echo signal (S R1) resulting from reflection of said first transmit signal by a surface of said product back towards said transceiver arrangement, the first transmission line probe comprising a first transmission line probe conductor and a first dielectric enclosing structure at least partly enclosing said first transmission line probe conductor along an entire length of the transmission line probe arranged inside said tank, so that said first transmission line probe provides a first coupling efficiency between said first transmit signal and the surrounding medium, to thereby guide said first transmit signal with a first propagation velocity exhibiting a first dependence on a dielectric constant of the surrounding medium; a second transmission line probe arranged and configured to guide an electromagnetic second transmit signal (S T2) from said transceiver arrangement through the surrounding medium towards said product inside the tank, and to return an electromagnetic second surface echo signal (S R2) resulting from reflection of said second transmit signal by the surface of said product back towards said transceiver arrangement, the second transmission line probe being configured to provide a second coupling efficiency between said second transmit signal and the surrounding medium, to thereby guide said second transmit signal with a second propagation velocity exhibiting a second dependence, different from the first dependence, on the dielectric constant of the surrounding medium; and processing circuitry connected to the transceiver arrangement for determining the filling level based on said first surface echo signal, said second surface echo signal, and a known relation between said first dependence and said second dependence on the dielectric constant of the surrounding medium.