Normalised radio flux density S_n(x) = S(v)/S, = x^αexp(−x⁻²¹) measurements (coloured open circles) as a function of the normalised frequency x = v/v_* for 17 SNRs spectra without low frequency turnover (see labels). This sample includes the spectra of G4.5+6.8 and G28.6-0.1 SNRs published in Castelletti et al. (2021), plus 15 new collected for this work. Extended lines show their power-law fit while dotted ones correspond to Eq. (2) with characteristic frequency, v_* (asterisk symbol) assigned as an upper limit according to the lowest measured frequency v_low. This normalised form sorts SNRs spectra in increasing order accordingly to their slope from bottom α = −0.69 to top α = −0.32. The black asterisk symbol marks the characteristic frequency, v_*, and highlights how the normalisation is done.