Rest-frame 1.4 GHz radio luminosity (L_{1.4 GHz}) vs. redshift for each XXL-S radio source with an optical counterpart (spectroscopic redshifts were used when available). The ATCA in-band spectral indices were used to convert the source flux densities from 2.1 GHz to 1.4 GHz. The red curve is the 5σ detection limit at 1.4 GHz, derived from the 2.1 GHz 5σ detection limit assuming a spectral index of −0.75.