A classic method for measuring the propagation constant of transmission lines is the one proposed by Bianco and Parodi more than four decades ago [1]. It is based on the invariance of the cross ratio of four complex numbers under a bilinear transformation. This means that it is possible to extract the value of the propagation constant (both real and imaginary parts) of an arbitrary transmission line (TL) by measuring, through an arbitrary (and unique) two-port, the input reflection coefficient of four TL sections of different lengths provided they are terminated by the same load. The accuracy of the original method can be greatly improved by measuring a higher number of TL sections and taking all the possible combinations of four lines.
