  We present multifrequency observations of the radio source 3CR 403.1, a
nearby (z=0.055), extended ($\sim$0.5 Mpc) radio galaxy hosted in a small
galaxy group. Using new high frequency radio observations from the Sardinia
Radio Telescope (SRT), augmented with archival low frequency radio
observations, we investigated radio spectral and polarimetric properties of 3CR
403.1. From the MHz-to-GHz spectral analysis, we computed the equipartition
magnetic field in the lobes to be B$_{eq}$=2.4~$\mu$G and the age of the source
to be $\sim$100 Myr. From the spectral analysis of the diffuse X-ray emission
we measured the temperature and density of the intracluster medium (ICM). From
the SRT observations, we discovered two regions where the radio flux density is
below the background value. We computed the Comptonization parameter both from
the radio and from the X-ray observations to test if the Sunyaev-Zel'dovich
effect is occurring here and found a significant tension between the two
estimates. If the negative signal is considered as real, then we speculate that
the discrepancy between the two values could be partially caused by the
presence of a non-thermal bath of mildly relativistic ghost electrons. From the
polarimetric radio images, we find a net asymmetry of the Faraday rotation
between the two prominent extended structures of 3CR 403.1, and constrain the
magnetic field strength in the ICM to be 1.8-3.5 $\mu$G. The position of 3CR
403.1 in the magnetic field-gas density plane is consistent with the trend
reported in the literature between central magnetic field and central gas
density.
