A radar level gauging system for determining at least one process variable related to a distance to a surface of a product in a tank comprising: transceiver circuitry arranged to generate a measurement signal, and to receive a reflected signal, processing circuitry connected to said transceiver circuitry and arranged to determine said process variable based on a relationship between said measurement signal and said reflected signal, a tubular wave guide arranged to extend into the product in the tank and to guide said measurement signals towards said surface, and to return said reflected signal, a radiator connected to said transceiver circuitry and arranged to emit said measurement signal as electromagnetic waves in an operating frequency range in an E-mode, and a mode generator adapted to convert electromagnetic waves emitted from said radiator from E-mode to H-mode and to convert electromagnetic waves returned from the tank from H-mode to E-mode, characterized in that said mode generator includes a lens in the form of a body transparent to electromagnetic waves in the operating frequency range, said lens being arranged in the tubular wave guide and having an optical axis aligned with a center axis of said tubular wave guide, said lens having a bottom surface facing the tank, which is adapted to transmit electromagnetic waves in H-mode and to reflect electromagnetic waves in E-mode, said lens having an upper surface opposite the bottom surface which is adapted to reflect electromagnetic waves and to turn an E-field of said electromagnetic waves upon reflection, so as to change a wave guide propagation mode of said electromagnetic waves from E-mode to H-mode or from H-mode to E-mode, said upper surface being transparent to electromagnetic waves in an area facing said radiator. transceiver circuitry arranged to generate a measurement signal, and to receive a reflected signal, processing circuitry connected to said transceiver circuitry and arranged to determine said process variable based on a relationship between said measurement signal and said reflected signal, a tubular wave guide arranged to extend into the product in the tank and to guide said measurement signals towards said surface, and to return said reflected signal, a radiator connected to said transceiver circuitry and arranged to emit said measurement signal as electromagnetic waves in an operating frequency range in an E-mode, and a mode generator adapted to convert electromagnetic waves emitted from said radiator from E-mode to H-mode and to convert electromagnetic waves returned from the tank from H-mode to E-mode, characterized in that said mode generator includes a lens in the form of a body transparent to electromagnetic waves in the operating frequency range, said lens being arranged in the tubular wave guide and having an optical axis aligned with a center axis of said tubular wave guide, said lens having a bottom surface facing the tank, which is adapted to transmit electromagnetic waves in H-mode and to reflect electromagnetic waves in E-mode, said lens having an upper surface opposite the bottom surface which is adapted to reflect electromagnetic waves and to turn an E-field of said electromagnetic waves upon reflection, so as to change a wave guide propagation mode of said electromagnetic waves from E-mode to H-mode or from H-mode to E-mode, said upper surface being transparent to electromagnetic waves in an area facing said radiator.