ATCA in-band radio spectral index (α_R) vs. full-band peak flux density (S_p,FB) for all XXL-S radio sources for which spectral index calculations are available (black points). The red circles with connecting red line show the median α_R values for each S_p,FB bin. The width of each S_p,FB bin is ~0.273 in log space. The median α_R for the faintest bin at S_p,FB ≈ 0.22 mJy is − 0.36, whereas at the brightest bin (S_p,FB ≈ 115 mJy) the median α_R is − 0.76. This clearly shows a trend toward flatter spectral indices for sources with S_p,FB ≲ 0.5 mJy.