  Magnetized plasmas traversed by linearly polarized light will reveal their
presence by the frequency dependent Faraday rotation of the angle of
polarization. The regions surrounding the black holes powering the jets in AGNs
are expected to have dense magnetized plasmas, possibly giving rise to very
large Faraday rotations. Compact steep spectrum (CSS) sources are good
candidates to search for very large Faraday rotated components as they contain
compact emission from close to the black hole and many are strongly depolarized
at centimeter wavelengths as expected from strong Faraday effects. We present
data on several CSS sources (3C48, 3C138 and 3C147) observed with the VLA at
frequencies between 20 and 48 GHz in the most extended configuration. Large,
but not excessive rotation measures are reported.
