Low-frequency (α_low) and high-frequency (α_high) radio spectral indices of the radio quasars discussed in this work. For clarity reasons, we only show values with an uncertainty on both spectral indices σ < 0.7. Sources with a single power law fit are shown on the 1:1 relation (dashed black line). Objects with a single power law fit are reported in this line. The dotted lines at α = 0, ±0.5 divide the space in different regions, as discussed in the text.