Abstract. We present multi-frequency radio observational results of the quasar 3C 48. The observations were carried out with the Very Large Array (VLA) at five frequencies, 0.33, 1.5, 4.8, 8.4, and 22.5 GHz, and with the Multi-Element Radio Linked Interferometer Network (MERLIN) at the two frequencies of 1.6 and 5 GHz. The source shows a one-sided jet to the north within 1 , which then extends to the northeast and becomes diffuse. Two bright components (N2 and N3), containing most of the flux density, are present in the northern jet. The spectral index of the two components is α N2 ∼ −0.99 ± 0.12 and α N3 ∼ −0.84 ± 0.23 (S ∝ ν α ). Our images show the presence of an extended structure surrounding component N2, suggestive of strong interaction between the jet and the interstellar medium (ISM) of the host galaxy. A steep-spectrum component, labelled S, located 0.25 southwest to the flat-spectrum component which could be the core of 3C 48, is detected at a significance of >15σ. Both the location and the steepness of the spectrum of component S suggest the presence of a counter-jet in 3C 48.

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???The definitive version is available at www3.interscience.wiley.com '. Copyright Royal Astronomical SocietyWe present results on the compact steep-spectrum quasar 3C 48 from observations with the Very Long Baseline Array (VLBA), the Multi-Element Radio Linked Interferometer Network (MERLIN) and the European Very long baseline interferometry (VLBI) Network (EVN) at multiple radio frequencies. In the 1.5-GHz VLBI images, the radio jet is characterized by a series of bright knots. The active nucleus is embedded in the southernmost VLBI component A, which is further resolved into two sub-components A1 and A2 at 4.8 and 8.3 GHz, respectively. A1 shows a flat spectrum and A2 shows a steep spectrum. The most strongly polarized VLBI components are located at component C ??? 0.25 arcsec north of the core, where the jet starts to bend to the north-east. The polarization angles at C show gradual changes across the jet width at all observed frequencies, indicative of a gradient in the emission-weighted intrinsic polarization angle across the jet and possibly a systematic gradient in the rotation measure; moreover, the percentage of polarization increases near the curvature at C, likely consistent with the presence of a local jet???interstellar-medium interaction and/or changing magnetic-field directions. The hot spot B shows a higher rotation measure, and has no detected proper motion. These facts provide some evidence for a stationary shock in the vicinity of B. Comparison of the present VLBI observations with those made 8.43 yr ago suggests a significant northward motion for A2 with an apparent transverse velocity ??app= 3.7 ?? 0.4c . The apparent superluminal motion suggests that the relativistic jet plasma moves at a velocity of ???0.96c if the jet is viewed at an inclination angle less than 20?? . A simple precessing jet model and a hydrodynamical isothermal jet model with helical-mode Kelvin???Helmholtz instabilities are used to fit the oscillatory jet trajectory of 3C 48 defined by the bright knot