  Multi-frequency radio polarimetry of the diffuse Galactic synchrotron
background gives new viewpoints on the Galactic magnetic field. Rotation
measure maps reveal magnetic structures on arcminute to degree scales, such as
a ring in polarization that we interpret as a magnetic tunnel. A complication
using this technique is depolarization across the beam and along the line of
sight. The influence of beam depolarization has been estimated using numerical
models of the magneto-ionic ISM, through which polarized radiation propagates.
The models show that depolarization canals similar to those observed can be
caused by beam depolarization, and that the one-dimensional gradients in RM
needed to produce these canals are ubiquitous in the medium.
