Rest-frame radio luminosity density at 150 MHz versus redshift for LOFAR radio-detected quasars (red) in our sample. The green (LOFAR radio-detected quasars) and gray squares (LOFAR radio-undetected quasars) are the radio luminosities obtained from the stacking analysis in Sect. 4.1. The dashed line denotes the 5σ flux limit (275 μJy) of the NDWFS-Boötes observations presented by Retana-Montenegro et al. (2018). The stacking analysis allows us to probe the radio-luminosity for quasars that are up to one order of magnitude fainter than the ones detected individually.