Normalised radio flux density S_n(x) = S(v)/S, = x^αexp(−x^−2.1) measurements (coloured filled circles) as a function of the normalised frequency x = v/v_* for 12 SNRs spectra with turnover (see labels). This sample comprises 9 SNR spectra reported in Castelletti et al. (2021), with the addition of three new spectra for the SNRs G189.1+3.0, G290.1–0.8, and G316.3+0.0 included for this work. Solid lines show their corresponding fit with parameters presented in our Table A.1. This normalised form sorts SNRs spectra in increasing order accordingly to their slope from bottom α = −0.89 to top α = −0.35. The black asterisk symbol marks the characteristic frequency v_* and highlight how the normalisation has been done.