Radio spectral indices measured in this work versus spectroscopic redshift. The spectral indices of known quasars compiled by Gloudemans et al. (2021, 2022) that are detected by LOFAR and either FIRST or VLASS are plotted for comparison. The sources from Fig. 1 without radio spectral index measurements are excluded. Our sample covers a wide range of spectral indices and contains 3 sources that could be classified as ultra-steep spectrum sources (α < −1.0).