This letter demonstrates that the conventional transmission/reflection method and the single-symmetric transmission line characterization are essentially equivalent to the evaluation of the eigensystems of the measured wave cascade matrix of a symmetric test line. This formulation is expressed through the even and odd reflection coefficients ( $\Gamma _{\text {e}}$ , $\Gamma _{\text {o}})$ of the symmetric test line, and it is not required to determine the signs of the square roots therein because of the passivity conditions of these coefficients. Furthermore, the passivity of the test line is conserved in the formulation. The improved formulation is verified theoretically and experimentally.