A radar level gauge system for determining a filling level of a product in a tank having a tubular mounting structure extending substantially vertically upwards from a roof of 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 arranged at an upper end of said mounting structure, said single conductor probe extending substantially vertically through said mounting structure past a lower end of said mounting structure towards and into the product in the tank for guiding an electromagnetic transmit signal (S T) from said transceiver through a tank atmosphere towards a surface of the product, and for returning an electromagnetic surface reflection signal (S R) resulting from reflection of said transmit signal at said surface back towards said transceiver; an electrically conductive shielding structure radially spaced apart from said single conductor probe and extending along a top portion of said single conductor probe inside said mounting structure and past the lower end of said mounting structure; and processing circuitry connected to said transceiver for determining said filling level based on said transmit signal and said surface reflection signal, wherein said shielding structure at least partly encloses the top portion of said single conductor probe, and is open in a radial direction to allow entry of said product, thereby enabling measurement of filling levels corresponding to the top portion of the probe, said shielding structure comprising a first portion extending along said single conductor probe inside the mounting structure, and a second portion extending along said single conductor probe below the lower end of the mounting structure, wherein the shielding structure exhibits, in a cross-section perpendicular to an extension of said single conductor probe a total enclosing arc angle (θ A) around said single conductor probe greater than 180° in said first portion, characterized in that the shielding structure exhibits, in a cross-section perpendicular to an extension of said single conductor probe a total enclosing arc angle (θ B-D) around said single conductor probe that decreases with increasing distance from the lower end of the mounting structure in said second portion. 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 arranged at an upper end of said mounting structure, said single conductor probe extending substantially vertically through said mounting structure past a lower end of said mounting structure towards and into the product in the tank for guiding an electromagnetic transmit signal (S T) from said transceiver through a tank atmosphere towards a surface of the product, and for returning an electromagnetic surface reflection signal (S R) resulting from reflection of said transmit signal at said surface back towards said transceiver; an electrically conductive shielding structure radially spaced apart from said single conductor probe and extending along a top portion of said single conductor probe inside said mounting structure and past the lower end of said mounting structure; and processing circuitry connected to said transceiver for determining said filling level based on said transmit signal and said surface reflection signal, wherein said shielding structure at least partly encloses the top portion of said single conductor probe, and is open in a radial direction to allow entry of said product, thereby enabling measurement of filling levels corresponding to the top portion of the probe, said shielding structure comprising a first portion extending along said single conductor probe inside the mounting structure, and a second portion extending along said single conductor probe below the lower end of the mounting structure, wherein the shielding structure exhibits, in a cross-section perpendicular to an extension of said single conductor probe a total enclosing arc angle (θ A) around said single conductor probe greater than 180° in said first portion, characterized in that the shielding structure exhibits, in a cross-section perpendicular to an extension of said single conductor probe a total enclosing arc angle (θ B-D) around said single conductor probe that decreases with increasing distance from the lower end of the mounting structure in said second portion.