Radio flux density measurements (coloured open circles) as a function of frequency for 17 SNRs spectra without turnovers (see labels). This sample includes the spectra of SNRs G4.5+6.8 and G28.6-0.1 published in Castelletti et al. (2021) plus 15 new ones constructed for this work. The solid coloured lines shows each corresponding power-law best fit, while the dotted ones correspond to Eq. (2) with a characteristic frequency v, (see asterisk symbol) assigned as an upper limit according to the lowest measured frequency, v_low.