The calculation of ε´ and εⁿ from the Roberts-von Hippel method is simplified for the cases where x₀ = 0 or ¼λ₁ and equations are developed for the calculation of these quantities. These equations are in the form given by Westphal for low loss materials, except for correction terms which differ slightly from unity. The correction terms are computed for various values of tan δ and m, a parameter depending on the sample thickness. Numerical evaluation of experimental data is carried out in a few cases to show the correctness of the simplified method of calculation.