The various physical parameters of a SNR can easily be worked out from graphs established on the basis of the recalibrated relation between the radio surface brightness Σ, and the linear diameterD of a SNR (Ilovaisky and Lequeux, 1972). These graphs lead to the estimation of the distancer (kpc), linear diameterD (pc), monochromatic power at 1 GHz,P 1 GHz (W Hz-1); and total powerP tot (a) (erg. s-1) of a SNR, given its mean angular diameter <ϑ> (arc min), flux density at 1 GHz, S1 GHz (f.u.) and spectral indexa. Three SNR (W28A1, Monoceros SNR, W49B) are used to illustrate the case. The radio spectrum of one of these (W49B), curved at low frequencies, is explained in terms of absorption by the diffuse interstellar medium. Various cases are discussed and some physical parameters of the absorbing matter are established.