Integrated Polarization of Sources at lambda ~1m and New Rotation
  Measure Ambiguities
  We present an analysis of the polarization of compact radio sources from six
pointings of the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz with
35% coverage in lambda^2. After correcting for the off-axis instrumental
polarization with a simple analytical model, only a small number of 585 strong
sources have significant polarizations at these wavelengths. The median
depolarization ratio from 1.4 GHz for the strongest sources is <0.2,
reinforcing the likelihood that radio galaxies are found in magnetized
environments, even outside of rich clusters. Seven sources with significant 350
MHz polarization were selected for a more in-depth Faraday structure analysis.
We fit the observed values Q/I and U/I as a function of lambda^2 using both a
depolarizing screen and two component models. We also performed RM
Synthesis/Clean and standard fitting of polarization angle vs. lambda^2. We
find that a single rotation measure (RM), as found using polarization angle
fitting or simple screen models, commonly provides a poor fit when the
solutions are translated back into Q, U space. Thus, although a single
"characteristic" rotation measure may be found using these techniques, the
Faraday structure of the source may not be adequately represented. We also
demonstrate that RM Synthesis may yield an erroneous Faraday structure in the
presence of multiple, interfering RM components, even when cleaning of the
Faraday spectrum is performed. We briefly explore the conditions under which
rotation measures and Faraday structure results can be reliable. Many
measurements in the literature do not meet these criteria; we discuss how these
influence the resulting scientific conclusions and offer a prescription for
obtaining reliable RMs.
