  Radio galaxies are linearly polarized -- an important property that allows us
to infer the properties of the magnetic field of the source and its
environment. However at low frequencies, Faraday rotation substantially
depolarizes the emission, meaning that comparatively few polarized radio
galaxies are known at low frequencies. Using the LOFAR Two Metre Sky Survey at
150 MHz and at 20 arcsec resolution, we select 342 radio galaxies brighter than
50 mJy and larger than 100 arcsec in angular size, of which 67 are polarized
(18 per cent detection fraction). These are predominantly Fanaroff Riley type
II (FR-II) sources. The detection fraction increases with total flux density,
and exceeds 50 per cent for sources brighter than 1 Jy. We compare the sources
in our sample detected by LOFAR to those also detected in NVSS at 1400 MHz, and
find that our selection bias toward bright radio galaxies drives a tendency for
sources depolarized between 1400 and 150 MHz to have flatter spectra over that
frequency range than those that remain polarized at 150 MHz. By comparing
observed rotation measures with an analytic model we find that we are
preferentially sensitive to sources in low mass environments. We also infer
that sources with one polarized hotspot are inclined by a small angle to the
line of sight, while sources with hotspots in both lobes lie in the plane of
the sky. We conclude that low frequency polarization in radio galaxies is
related to a combination of environment, flux density and jet orientation.
