Faraday spectrum of the off-axis source 4C +66.19 extracted from the 85 cm RM cube. The flux in in units of mJy beam^-1 RMSF^-1. The resonances at the value  |φ|   ~ 42 rad m^-2 come from to the 17 MHz periodicity of the Q and U signals for off-axis sources (see text). RM-cleaning has been applied.