  We present an in-depth study of the large-scale, western jet of the
microquasar XTE J1550-564, based on X-ray and radio observations performed in
2002-2003. The jet is spatially resolved in both observing windows. The X-ray
jet is expanding in time along the axis of the jet's propagation: we observe
the formation of a tail (~2.25"), which appears to extend backwards with an
apparent velocity ~-0.10c. The origin of this feature is discussed in the
framework of scenarios of energy dissipation. A single power-law adequately
describes the broadband spectra, supporting a synchrotron origin of the X-ray
emission. However, a spectral break at ~10^{15} Hz is necessary in coincidence
with a re-flare at 8.64 GHz in September 2002. This finding may be indicative
of emission from newly accelerated low-energy particles. The first detection of
the jet is in February 2001 (F_{8.64GHz}=0.25+/-0.09 mJy) in the flux rising
phase. A phase of stable emission is followed by a rapid decay
(t_{decay}=167+/-5 days). The decay at radio frequencies is significantly
shorter than in X-rays (t_{decay}=338+/-14 days). We detected a high fraction
(up to ~9%) of linearly polarized radiation at 4.8 GHz and 8.6 GHz. The
orientation of the electric vector is consistent with the picture of a
shock-compressed magnetic field, and there are hints of variations on
month-timescales, possibly connected with the evolution of the jet structure.
