Black histogram: distribution of FWHM (in decades of frequency) of 14 FR 0s with a convex radio spectrum extracted from 19 sources of bright sub-sample with F_1.4 >  50 mJy. The left pointing arrows correspond to upper limits of the FWHM due to the non detection at least at one frequency. The right pointing arrow represents the FWHM of J1604+17 whose measured value (12.5) exceeds the plot limit. The dashed red histogram reports the FWHM measured by O’Dea et al. (1991) in a sample of 13 GPS.