X-ray-to-UV relative emission (${\stackrel{\sim }{\alpha }}_{\mathrm{ox}}$ parameter) of the RACS sources with X-ray coverage as a function of their redshift. The dotted red line shows the median value of the RACS sources with an X-ray detection, while the dashed grey line is the median value of the blazar sample analysed in Ighina et al. (2019). The dashed blue area showed range of values expected from the X-ray corona (i.e. from a radio-quiet quasar) with a monochromatic UV luminosity similar to the RACS sample, log$L_{2500\mathrm{\AA }}=30.7-32.0$ erg s⁻¹ Hz⁻¹, based on the relation described in Lusso & Risaliti (2016) and assuming a photon index in the range Γ = 1.7 − 2.1.