  We present a detailed study of the Faraday depth structure of four bright (>
1 Jy), strongly polarized, unresolved, radio-loud quasars. The Australia
Telescope Compact Array (ATCA) was used to observe these sources with 2 GHz of
instantaneous bandwidth from 1.1 to 3.1 GHz. This allowed us to spectrally
resolve the polarization structure of spatially unresolved radio sources, and
by fitting various Faraday rotation models to the data, we conclusively
demonstrate that two of the sources cannot be described by a simple rotation
measure (RM) component modified by depolarization from a foreground Faraday
screen. Our results have important implications for using background
extragalactic radio sources as probes of the Galactic and intergalactic
magneto-ionic media as we show how RM estimations from narrow-bandwidth
observations can give erroneous results in the presence of multiple interfering
Faraday components. We postulate that the additional RM components arise from
polarized structure in the compact inner regions of the radio source itself and
not from polarized emission from Galactic or intergalactic foreground regions.
We further suggest that this may contribute significantly to any RM
time-variability seen in RM studies on these angular scales. Follow-up,
high-sensitivity VLBI observations of these sources will directly test our
predictions.
