  Multi-frequency radio polarimetric observations of the diffuse Galactic
synchrotron background enable us to study the structure of the diffuse ionized
gas via rotation measure maps. However, depolarization will introduce artifacts
in the resulting rotation measure, most notably in the form of narrow,
elongated ``depolarization canals''. We use numerical models of a non-emitting
Faraday rotating medium to study the RM distribution needed to create
depolarization canals by depolarization due to a finite beam width, and to
estimate the influence of this depolarization mechanism on the determination of
RM. We argue that the depolarization canals indeed can be caused by beam
depolarization, which in turn is a natural consequence when observing a
turbulent medium with limited resolution. Furthermore, we estimate that beam
depolarization can induce an additional error of about 20% in RM
determinations, and considerably less in regions that are not affected by
depolarization canals.
