Spectral index distribution of the radio-AGN galaxies with both LoTSS and 1.4 GHz measurements, as a function of 1.4 GHz flux density. Individual sources are shown as blue dots and the medians of different bins in 1.4 GHz flux density are shown as orange crosses. The vertical lines correspond to the standard deviation on each bin. The median spectral index at flux densities S_1.4 GHz > 20 mJy is 0.63. The weak decrease in median spectral index at lower flux densities may be driven by selection biases in the 1.4 GHz sample, as indicated by the black dashed line that simulates the effect of this (see text for details).