A tank arrangement including a guided wave radar level gauge installed in a tank, and having a single wire transmission line probe extending through a passage through a conducting structure in the tank. Along the section of the probe that extends through the passage, the arrangement comprises a propagation field limiting structure adapted to reduce a propagation field of an electromagnetic signal propagating along the probe. With this design, the radial extension of the propagating field can be locally reduced so that a sufficient portion of the signal power is allowed to pass through the passage.