I developed a new approach to determine the matched loss of transmission lines. This approach is particularly suited for balanced parallel-wire transmission lines. Their attenuation can not be measured directly in a 50-Ω-system because their line impedance differs considerably from 50 Ω.
The new approach is based on a S11-measurement of the transmission line under test using a calibrated vector network analyzer. The end of the line remains open. In the second step the line attenuation at the line resonances of λ/4 and their odd multiples is calculated.
Details can be found in my article (in German) "Dämpfung und Verkürzungsfaktor von Zweidrahtleitungen", FUNKAMATEUR 65 (2016), Heft 11, S. 1034-1039.
The imaginary part of S11 is zero at the line resonances, the corresponding real part has to be put in. Furthermore the line impedance ZL of the line under test (e.g. from data sheet) and the mechanical line length are needed. The script calculates the impedance at the beginning of the line, the line attenuation in dB and the attenuation constant α in dB/100m.