A radar level gauge system for determining a filling level of a product in a tank, said radar level gauge system comprising: a transceiver configured to generate and transmit electromagnetic signals having a first polarization and to receive electromagnetic signals having a second polarization different from the first polarization; a horn antenna for directing an electromagnetic transmit signal (S T) transmitted by said transceiver towards said surface of the product, and for returning back to said transceiver an electromagnetic surface reflection signal (S R) resulting from reflection of said electromagnetic transmit signal at said surface; and processing circuitry connected to said transceiver for determining said filling level based on said electromagnetic surface reflection signal, wherein said horn antenna comprises: a first opening coupled to said transceiver; a second opening, larger than said first opening, directed towards said surface of said product in the tank; and a conductive horn antenna wall connecting said first opening and said second opening to form a horn structure having a central longitudinal axis, wherein said conductive horn antenna wall exhibits an electrical distance, as a function of a rotational orientation in respect of said central longitudinal axis of a half-plane extending from said central longitudinal axis, from said first opening to said second opening, along a path defined by an intersection between said conductive horn antenna wall and said half-plane, characterized in that said electrical distance as a function of said rotational orientation exhibits: a first maximum and a second maximum located opposite each other with respect to said central longitudinal axis; and a first minimum and a second minimum located opposite each other with respect to said central longitudinal axis; and in that: the electrical distance for said first maximum is substantially the same as the electrical distance for said second maximum; the electrical distance for said first minimum is substantially the same as the electrical distance for said second minimum; and a difference in electrical distance for said first maximum and said first minimum is a quarter of a wavelength of said electromagnetic transmit signal (S T). a transceiver configured to generate and transmit electromagnetic signals having a first polarization and to receive electromagnetic signals having a second polarization different from the first polarization; a horn antenna for directing an electromagnetic transmit signal (S T) transmitted by said transceiver towards said surface of the product, and for returning back to said transceiver an electromagnetic surface reflection signal (S R) resulting from reflection of said electromagnetic transmit signal at said surface; and processing circuitry connected to said transceiver for determining said filling level based on said electromagnetic surface reflection signal, wherein said horn antenna comprises: a first opening coupled to said transceiver; a second opening, larger than said first opening, directed towards said surface of said product in the tank; and a conductive horn antenna wall connecting said first opening and said second opening to form a horn structure having a central longitudinal axis, a first opening coupled to said transceiver; a second opening, larger than said first opening, directed towards said surface of said product in the tank; and a conductive horn antenna wall connecting said first opening and said second opening to form a horn structure having a central longitudinal axis, wherein said conductive horn antenna wall exhibits an electrical distance, as a function of a rotational orientation in respect of said central longitudinal axis of a half-plane extending from said central longitudinal axis, from said first opening to said second opening, along a path defined by an intersection between said conductive horn antenna wall and said half-plane, characterized in that said electrical distance as a function of said rotational orientation exhibits: a first maximum and a second maximum located opposite each other with respect to said central longitudinal axis; and a first minimum and a second minimum located opposite each other with respect to said central longitudinal axis; and a first maximum and a second maximum located opposite each other with respect to said central longitudinal axis; and a first minimum and a second minimum located opposite each other with respect to said central longitudinal axis; and in that: the electrical distance for said first maximum is substantially the same as the electrical distance for said second maximum; the electrical distance for said first minimum is substantially the same as the electrical distance for said second minimum; and a difference in electrical distance for said first maximum and said first minimum is a quarter of a wavelength of said electromagnetic transmit signal (S T). the electrical distance for said first maximum is substantially the same as the electrical distance for said second maximum; the electrical distance for said first minimum is substantially the same as the electrical distance for said second minimum; and a difference in electrical distance for said first maximum and said first minimum is a quarter of a wavelength of said electromagnetic transmit signal (S T).