A radar level gauge system for gauging the level of a filling material in a container is disclosed. The system comprises a waveguide extending towards the surface of said filling material; a transmitter for transmitting a microwave signal of a first mode of propagation in the waveguide; and a receiver for receiving the microwave signal reflected against the surface of the filling material and propagating back through the waveguide. Further, it comprises a processing circuitry for determining the filling level of the container based on the reflected microwave signal; and a transition element connecting the waveguide and the transmitter, wherein the transition element is configured to allow a part of the transmitted microwave signal to leak into a second mode of propagation. The first and second modes of propagation are within a frequency band which admits propagation of said microwave signal in the two different modes of propagation in the waveguide, wherein the receiver is arranged to receive the microwave signal in the at least two different modes of propagation.
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