Radio flux density measurements (coloured filled circles) as a function of frequency for 12 SNRs spectra with turnovers (see labels). This sample is built of 9 SNR spectra reported in Castelletti et al. (2021) plus 3 new spectra for the SNRs G189.1+3.0, G290.1–0.8, and G316.3+0.0, which were specifically constructed for this work. The solid coloured lines shows each corresponding best fit given by Eq. (2) with parameters included in our Table A.1. The asterisk symbol indicates the individual characteristic frequency, v_*