A radar level gauge using electromagnetic waves for determining a filling level of a product in a tank, said radar level gauge comprising: a transceiver for transmitting an electromagnetic transmit signal and receiving an electromagnetic echo signal; processing circuitry connected to said transceiver for determining said filling level based on said electromagnetic echo signal; a signal propagating device adapted to direct said electromagnetic transmit signal towards a surface of said product and to return a reflection from said surface as said electromagnetic echo signal, and a wave guide arrangement, connecting said transceiver with said signal propagation device, said wave guide arrangement including: a hollow wave guide, comprising an inner portion formed by a first electrically conducting wave guide forming member, and an outer portion formed by a second electrically conducting wave guide forming member (38, 43), and a first dielectric wave guide filling member, having an elongated center portion with an inner end received in the inner wave guide portion and an outer end received in the outer wave guide portion, and a cup-shaped sleeve portion (33, 34) extending radially outwards from said center portion and axially along an axis of said center portion, said sleeve thus forming a cup-shape with an opening facing away from said tank, at least part of said cup-shaped sleeve portion being sandwiched between said first and second wave guide forming members, and at least one channel providing fluid contact between an outer rim of the cup-shaped sleeve portion to an exterior of the tank. a transceiver for transmitting an electromagnetic transmit signal and receiving an electromagnetic echo signal; processing circuitry connected to said transceiver for determining said filling level based on said electromagnetic echo signal; a signal propagating device adapted to direct said electromagnetic transmit signal towards a surface of said product and to return a reflection from said surface as said electromagnetic echo signal, and a wave guide arrangement, connecting said transceiver with said signal propagation device, said wave guide arrangement including: a hollow wave guide, comprising an inner portion formed by a first electrically conducting wave guide forming member, and an outer portion formed by a second electrically conducting wave guide forming member (38, 43), and a first dielectric wave guide filling member, having an elongated center portion with an inner end received in the inner wave guide portion and an outer end received in the outer wave guide portion, and a cup-shaped sleeve portion (33, 34) extending radially outwards from said center portion and axially along an axis of said center portion, said sleeve thus forming a cup-shape with an opening facing away from said tank, at least part of said cup-shaped sleeve portion being sandwiched between said first and second wave guide forming members, and a hollow wave guide, comprising an inner portion formed by a first electrically conducting wave guide forming member, and an outer portion formed by a second electrically conducting wave guide forming member (38, 43), and a first dielectric wave guide filling member, having an elongated center portion with an inner end received in the inner wave guide portion and an outer end received in the outer wave guide portion, and a cup-shaped sleeve portion (33, 34) extending radially outwards from said center portion and axially along an axis of said center portion, said sleeve thus forming a cup-shape with an opening facing away from said tank, at least part of said cup-shaped sleeve portion being sandwiched between said first and second wave guide forming members, and at least one channel providing fluid contact between an outer rim of the cup-shaped sleeve portion to an exterior of the tank.