Normalised radio flux density measurements as a function of frequency for the SNRs G4.5+6.8 and G28.6–0.1 presented in Castelletti et al. (2021) plus 15 new SNR spectra constructed for this work (coloured open circles). These datasets are complemented by flux density estimates from Kovalenko et al. (1994b) as black open squares and Kassim (1989) as black open triangles. In order to highlight the drop-off behavior, the data are normalised to the power-law emission E(x) = x^α, where x = v/v_*. The curved solid line is the normalised absorption A(x) = exp(−x^−2.1). The total rms of the coloured open circles is less than 15%.