Left: Spectral index against the LOFAR integrated flux density for all common Apertif and LOFAR sources. The markers show the median spectral index in a given flux density interval. The solid line indicates a lower limit calculated for the Apertif completeness level of 0.3 mJy (Sect. 4.1). Right: Luminosity against redshift for the main sample. The solid lines indicate a Malmquist bias-free region (Sect. 4.2).