Online Material

Table 2: Flux densities and polarised flux densities from Effelsberg 100-m and NVSS measurements.

Table 3: Percentage of polarised flux density and position angle of the electric vector at the five frequencies.

Table 4: Depolarisation indices and RMs.

Appendix 1

In this Appendix, we present plots of the fractional polarisation m derived form our Effelsberg observations complemented with those extracted from the NVSS at 1.4 GHz and those taken from Tabara & Inoue (1980). The values of m predicted by the models of Burn and Tribble are shown for comparison. Also plotted are polarisation angles versus $\lambda ^2$ with their corresponding linear best fit.

$\begin{figure} \par\includegraphics[width=8cm,clip]{11815a02.ps}\includegraphics[width=8cm,clip]{11815b02.ps} \end{figure}$	Figure A.1: Position angles of the electric field vector $\chi$ in deg (dots) and fractional polarisation m in % (triangles) versus $\lambda ^2$ in m² for the source 3C 43, for the full range ( left), and for a narrow range ( right) of wavelengths. The solid line represents the Tribble model, the dashed line the Burn model.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{11815a03.ps}\includegraphics[width=8cm,clip]{11815b03.ps} \end{figure}$	Figure A.2: Position angles $\chi$ and fractional polarisation m for the source 3C 48. Layout as in Fig. A.1.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{11815a04.ps} \end{figure}$	Figure A.3: Position angles $\chi$ and fractional polarisation m for the source 3C 67. Layout as in Fig. A.1.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{11815a05.ps} \end{figure}$	Figure A.4: Position angles $\chi$ and fractional polarisation m for the source 0319+12. Layout as in Fig. A.1.
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$\begin{figure} \addtocounter{figure}{+0} \par\includegraphics[width=8cm,clip]{11815a06.ps}\includegraphics[width=8cm,clip]{11815b06.ps} \end{figure}$	Figure A.5: Position angles $\chi$ and fractional polarisation m for the source 3C 119. Layout as in Fig. A.1.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{11815a07.ps}\includegraphics[width=8cm,clip]{11815b07.ps} \end{figure}$	Figure A.6: Position angles $\chi$ and fractional polarisation m for the source 3C 138. Layout as in Fig. A.1.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{11815a08.ps} \end{figure}$	Figure A.7: Position angles $\chi$ and fractional polarisation m for the source 3C 268.3. Layout as in Fig. A.1.
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$\begin{figure} \par\includegraphics[width=7.7cm,clip]{11815a09.ps}\includegraphics[width=7.7cm,clip]{11815b09.ps} \end{figure}$	Figure A.8: Position angles $\chi$ and fractional polarisation m for the source 3C 277.1. Layout as in Fig. A.1.
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$\begin{figure} \includegraphics[width=7.7cm,clip]{11815a10.ps}\includegraphics[width=7.7cm,clip]{11815b10.ps} \end{figure}$	Figure A.9: Position angles $\chi$ and fractional polarisation m for the source 3C 287. Layout as in Fig. A.1.
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$\begin{figure} \includegraphics[width=7.7cm,clip]{11815a11.ps}\includegraphics[width=7.7cm,clip]{11815b11.ps} \end{figure}$	Figure A.10: Position angles $\chi$ and fractional polarisation m for the source 3C 298. Layout as in Fig. A.1.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{11815a12.ps} \end{figure}$	Figure A.11: Position angles $\chi$ and fractional polarisation m for the source 1442+10. Layout as in Fig. A.1.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{11815a13.ps}\includegraphics[width=8cm,clip]{11815b13.ps} \end{figure}$	Figure A.12: Position angles $\chi$ and fractional polarisation m for the source 3C 309.1. Layout as in Fig. A.1.
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$\begin{figure} \addtocounter{figure}{+0} \par\includegraphics[width=8cm,clip]{11815a14.ps}\includegraphics[width=8cm,clip]{11815b14.ps} \end{figure}$	Figure A.13: Position angles $\chi$ and fractional polarisation m for the source 3C 318. Layout as in Fig. A.1.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{11815a15.ps}\includegraphics[width=8cm,clip]{11815b15.ps} \end{figure}$	Figure A.14: Position angles $\chi$ and fractional polarisation m for the source 4C 11.69. Layout as in Fig. A.1.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{11815a16.ps}\includegraphics[width=8cm,clip]{11815b16.ps} \end{figure}$	Figure A.15: Position angles $\chi$ and fractional polarisation m for the source 3C 454. Layout as in Fig. A.1.
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$\begin{figure} \par\includegraphics[width=8cm,clip]{11815a17.ps}\includegraphics[width=8cm,clip]{11815b17.ps} \end{figure}$	Figure A.16: Position angles $\chi$ and fractional polarisation m for the source 3C 455. Layout as in Fig. A.1.
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Appendix 2

2.1 Sources following the Tribble model

The model proposed by Tribble reproduces the data of about one third of our sample, namely of 3C 48, 3C 67, 3C 119, 3C 138, 3C 268.3, 3C 277.1, and 3C 318. Sources such as 3C 48, 3C 67, 3C 138, and 3C 277.1 also show RM $< \la$ few 10 rad m^-2, which is an indication of an unresolved foreground screen, presumably the halo of our own Galaxy.

The source 3C 119, which has a very high RM and thus a fast decline in its fractional polarisation, might already be affected by significant depolarisation at wavelengths shorter than 2.8 cm. A higher m₀ would lead to depolarisation according to the Tribble law.

2.2 Sources with an indication of repolarisation

Eight sources (3C 43, 0319+12, 3C 287, 1442+10, 3C 309.1, 4C 11.69, 3C 454, and 3C 455) show indications of repolarisation, i.e., an increase in fractional polarisation with decreasing frequency, generally at short wavelengths $\la$ 10 cm; this corresponds to a relatively strong increase in fractional polarisation followed by a decline that can still be described by the Tribble law. This effect is visible despite possible instrumental effects of different telescopes and also considering that observations were made at different epochs. In three of these sources (3C 309.1, 4C 11.69, 3C 454), the repolarisation effect is indeed confused with possible time variability. Focusing on our simultaneous measurements only, we still find constant or slightly increasing fractional polarisation with increasing wavelength, which is not predicted by any depolarisation model. In particular, the galaxy 3C 455 shows a measured repolarisation greater than the 3 $\sigma$ level between 10.45 GHz and 2.64 GHz together with a small value of theRM $_{\rm rf}$ (194 rad m^-2), which is an indication of the influence of a foreground screen, possibly an extended cloud with [OII] emission detected by Hes et al. (1996). A plausible mechanism would be the effect of shear layers caused by the interaction between the surface of an expanding source and the surrounding medium (Burn 1966).

The source 3C 455 was observed with the VLA at 8.35 GHz by Bogers et al. (1994). It shows a triple structure with the indication of a jet joining the core with the south western lobe. The three components are almost aligned along the source major axis extending up to about 4 $\hbox{$^{\prime\prime}$ }$ . However, 3C 455 appears slightly resolved by the NVSS, which has a restoring beam of 45 $\hbox{$^{\prime\prime}$ }$ , suggesting that the three components imaged by Bogers et al. are actually embedded in a more extended region of low brightness emission. This can be seen in the image available in the VLA Low-Frequency Sky Survey (VLSS; Cohen et al. 2007) at 74 MHz, which shows an even more extended structure of about 2 $\hbox{$^\prime$ }$ in size. The existence of this extended emission is supported by the source spectral index, which indicates an upturn towards higher flux density above $\sim$ 100 MHz. Therefore, a second possible interpretation is that by observing at 2.64 GHz or lower frequencies, we have integrated the flux density and polarised flux density from that region. At 10.45 GHz, the steep spectrum extended structure is below the detection limit.

A similar case of repolarisation at a lower frequency was pointed out by Montenegro-Montes et al. (2008) for the source 1159+01.

2.3 Polarisation variability

Finally, 3C 298 could exhibit time variability in its fractional polarisation. Unfortunately, the current data base does not provide a sufficient number of simultaneous measurements to prove this effect, which was reported for example by Aller et al. (2003) for the source 3C 147.