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, processing circuitry, a wave guiding structure arranged guide measurement signals towards the surface, and a radiator connected to the transceiver circuitry and arranged to emit the measurement signal into the wave guiding structure. The system further comprises a mode generator adapted to convert electromagnetic waves emitted from the radiator from a first propagation mode to a second propagation mode, wherein the mode generator includes a lens in the form of a body transparent to electromagnetic waves in the operating frequency range.With this design, electromagnetic waves having the first propagation mode emitted by the radiator into the lens will be reflected at least twice within the lens (first in the bottom surface and then in the upper surface). At the second reflection (in the upper surface of the lens), the propagation mode will be changed, and the electromagnetic waves exiting the lens will have the second propagation mode.