The uncertainty of measurements of the complex permittivity epsilon(r) and permeability mu(r) as a function of the normalized sample length l/LAMBDA' due to the measured errors in the scattering coefficients S11 and S21 is discussed. A simple analysis of the expressions, a numerical analysis for several values of epsilon(r) and mu(r) and some measured data are given, which allows us to determine the extent of l/LAMBDA' and maximum uncertainty of the combined total transmission-reflection method in application to radar absorber measurements.