in Section \[section:pm\].

image analysis
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Spectral index distribution along the radio jet
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In order to measure the spectral properties of the 3C 48 jet, we re-mapped the 4.99-GHz MERLIN data acquired on 1992 June 15 [@Feng05] and compared it with the 1.65-GHz EVN+MERLIN data described in the present paper. The individual data sets were first mapped with the same [*uv*]{} range, and convolved with the same 40$\times$40 (mas) restoring beam. Then we compared the intensities of the two images pixel by pixel to calculate the spectral index $\alpha^{4.99}_{1.65}$. The results are shown in Figure \[fig:spix\]. Component A shows a rather flat spectrum with a spectral index $\alpha^{4.99}_{1.65}=-0.24\pm0.09$. All other bright knots show steep spectral indices, ranging from $-0.66$ to $-0.92$. The extended envelope in general has an even steeper spectrum with $\alpha\lesssim-1.10$. Spectral steepening in radio sources is a signature of a less efficient acceleration mechanism and/or the depletion of high-energy electrons through synchrotron/Compton radiation losses and adiabatic losses as a result of the expansion of the plasma as it flows away from active acceleration region. The different spectral index distribution seen in the compact jet and extended envelope may indicate that there are different electron populations in 