It has been reported recently that in many extended high-luminosity extragalactic radio sources, the lobe on the jet side shows less depolarization than the lobe on the counterjet side. This depolarization asymmetry has been interpreted as being produced by an external foreground screen. Depolarization caused by such an external medium supports the idea that the jets are relativistically beamed, since higher depolarization is expected on the counterjet side. In this regard, the one-sided jet quasar 3C 47 was observed with the VLA at 6 and 20 cm. We derived maps of the projected magnetic field, depolarization, and rotation measure. Significant differences for these parameters have been found among the various components of the source. In particular, 3C 47 shows a Strong depolarization asymmetry between the NE (counterjet side) and the SW (jet side). The distribution of the depolarization ratio is quite uniform in each lobe, but quite asymmetric between the two lobes. We argue here that this is likely to be a signature of a foreground screen. The consequences of this model for the relativistic beaming hypothesis are also explored.