the 3C 48 data. After correction of instrumental polarization and absolute polarization angle, the cross-correlated 3C 48 data were used to produce Stokes $Q$ and $U$ images, from which maps of linear polarization intensity and position angle were produced.

MERLIN observations and data reduction
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The MERLIN observations of 3C 48 were performed in the fake-continuum mode: the total bandwidth of 15 MHz was split into 15 contiguous channels, 1 MHz for each channel. A number of strong, compact extragalactic sources were interspersed into the observations of 3C 48 to calibrate the complex antenna gains.

The MERLIN data were reduced in [aips]{} following the standard procedure described in the MERLIN cookbook. The flux-density scale was determined using 3C 286 which has a flux density of 13.7 Jy at 1.65 GHz. The phases of the data were corrected for the varying parallactic angles on each antenna. Magnetized plasma in the ionosphere results in an additional phase difference between the right- and left-handed signals, owing to Faraday rotation. This time-variable Faraday rotation tends to defocus the polarized image and to give rise to erroneous estimates of the instrumental polarization parameters. We estimated the ionospheric Faraday rotation on each antenna based on the 