A tank arrangement comprising: a tank having a nozzle; and a radar level gauge system, for determining a filling level of a product contained in said tank, said radar level gauge system comprising: a transceiver arranged outside said tank for generating, transmitting and receiving electromagnetic signals; a single conductor probe arranged inside said tank and electrically connected to said transceiver via a feed-through (15; 19) arranged at the top of the nozzle of said tank, said single conductor probe extending through the nozzle towards and into said product contained in the tank, for guiding a transmitted signal (S T) from said transceiver towards a surface of said product, and for returning a surface echo signal (S R) resulting from reflection of said transmitted signal at said surface back towards said transceiver; an impedance matching arrangement (35; 40; 44; 48) provided to the probe and extending along a portion of the probe; and processing circuitry connected to said transceiver for determining said filling level based on said surface echo signal, wherein an extension of said impedance matching arrangement (35; 40; 44; 48), in a direction perpendicular to said probe changes with increasing distance from the feed-through (15; 19) with a second, negative rate of change below said nozzle over a distance corresponding to at least a quarter of a wavelength of the transmitted signal (S T), characterized in that: the radial extension of said impedance matching arrangement (35; 40; 44; 48) is constant or changes with increasing distance from the feed-through (15; 19) with a first rate of change inside said nozzle, said first rate of change being more positive than said second rate of change; and said impedance matching arrangement, inside said nozzle, has a sufficiently large radial extension to remove or at least significantly reduce an influence of said nozzle on said transmitted signal. a tank having a nozzle; and a radar level gauge system, for determining a filling level of a product contained in said tank, said radar level gauge system comprising: a transceiver arranged outside said tank for generating, transmitting and receiving electromagnetic signals; a single conductor probe arranged inside said tank and electrically connected to said transceiver via a feed-through (15; 19) arranged at the top of the nozzle of said tank, said single conductor probe extending through the nozzle towards and into said product contained in the tank, for guiding a transmitted signal (S T) from said transceiver towards a surface of said product, and for returning a surface echo signal (S R) resulting from reflection of said transmitted signal at said surface back towards said transceiver; an impedance matching arrangement (35; 40; 44; 48) provided to the probe and extending along a portion of the probe; and processing circuitry connected to said transceiver for determining said filling level based on said surface echo signal, wherein an extension of said impedance matching arrangement (35; 40; 44; 48), in a direction perpendicular to said probe changes with increasing distance from the feed-through (15; 19) with a second, negative rate of change below said nozzle over a distance corresponding to at least a quarter of a wavelength of the transmitted signal (S T), characterized in that: the radial extension of said impedance matching arrangement (35; 40; 44; 48) is constant or changes with increasing distance from the feed-through (15; 19) with a first rate of change inside said nozzle, said first rate of change being more positive than said second rate of change; and said impedance matching arrangement, inside said nozzle, has a sufficiently large radial extension to remove or at least significantly reduce an influence of said nozzle on said transmitted signal. a transceiver arranged outside said tank for generating, transmitting and receiving electromagnetic signals; a single conductor probe arranged inside said tank and electrically connected to said transceiver via a feed-through (15; 19) arranged at the top of the nozzle of said tank, said single conductor probe extending through the nozzle towards and into said product contained in the tank, for guiding a transmitted signal (S T) from said transceiver towards a surface of said product, and for returning a surface echo signal (S R) resulting from reflection of said transmitted signal at said surface back towards said transceiver; an impedance matching arrangement (35; 40; 44; 48) provided to the probe and extending along a portion of the probe; and processing circuitry connected to said transceiver for determining said filling level based on said surface echo signal, the radial extension of said impedance matching arrangement (35; 40; 44; 48) is constant or changes with increasing distance from the feed-through (15; 19) with a first rate of change inside said nozzle, said first rate of change being more positive than said second rate of change; and said impedance matching arrangement, inside said nozzle, has a sufficiently large radial extension to remove or at least significantly reduce an influence of said nozzle on said transmitted signal.