Top panel: Distribution of the α_low and α_high parameters derived for all the radio quasars discussed in this work. In case of a single power law fit, the spectral index is considered in both distributions. Dotted vertical lines indicate the median values of each spectral index distribution (0.32 and 0.45 for α_low and α_high, respectively). The best-fit Gaussian distributions are shown with dotted black lines. Bottom panel: Distribution of the break frequency in the observed frame for all the sources for which a double power law was favoured. The dotted vertical line is the median value (1.3 GHz). In both panels, the distribution of z > 5 sources is highlighted with filled histograms.