the absolute EVPAs at 8.3 GHz.

EVN observations and data reduction
-----------------------------------

The effective observing time on 3C 48 was about 8 hours. Apart from occasional RFI (radio frequency interference), the whole observation ran successfully. The data were recorded in four IFs. Each IF was split into 16 channels, each of 0.5-MHz channel width. In addition to 3C 48 we observed the quasars DA 193 and 3C 138 for phase calibration. 3C 138 was used as a fringe finder due to its high flux density of $\sim$9 Jy at 1.65 GHz.

The amplitude of the visibility data was calibrated using the system temperatures, monitored during the observations, and gain curves of each antenna that were measured within 2 weeks of the observations. The parallactic angles were determined on each telescope and the data were corrected appropriately before phase and polarization calibration. We corrected the ionospheric Faraday rotation using archival ionospheric model data from the CDDIS. DA 193 and OQ 208 were used to calibrate the complex bandpass response of each antenna. We first ran fringe fitting on DA 193 over a 10-minute time span to align the multi-band delays. Then a full fringe fitting using all calibrators over the whole 