  The polarized component of the diffuse radio synchrotron emission of our
Galaxy shows structure, which is apparently unrelated to the structure in total
intensity, on many scales. The structure in the polarized emission can be due
to several processes or mechanisms. Some of those are related to the
observational setup, such as beam depolarization -- the vector combination and
(partial) cancellation of polarization vectors within a synthesized beam --, or
the insensitivity of a synthesis telescope to structure on large scales, also
known as the 'missing short spacings problem'. Other causes for structure in
the polarization maps are intrinsic to the radiative transfer of the emission
in the warm ISM, which induces Faraday rotation and depolarization.
  We use data obtained with the Westerbork Synthesis Radio Telescope at 5
frequencies near 350 MHz to estimate the importance of the various mechanisms
in producing structure in the linearly polarized emission. In the two regions
studied here, which are both at positive latitudes in the second Galactic
quadrant, the effect of 'missing short spacings' is not important. The
properties of the narrow depolarization 'canals' that are observed in abundance
lead us to conclude that they are mostly due to beam depolarization, and that
they separate regions with different rotation measures. As beam depolarization
only creates structure on the scale of the synthesized beam, most of the
structure on larger scales must be due to depth depolarization. We do not
discuss that aspect of the observations here, but in a companion paper we
derive information about the properties of the ISM from the structure of the
polarized emission.
