Hi, I've just been reading the famous ENGEN paper on TRL calibration and was wondering why I need to specify the line delay at the GM of my frequency band in my Cal Kit when the TRL calibration itself inherently determines this delay and I already know the physical length of the line? I am using dispersive transmission lines on an 8510. Sorry if the question sounds stupid but I have not been able to get my head round it so a most basic explanation will be welcome.

Another way of thinking about it is that TRL can determine S21*S12. Assume S21=S12, then to get S21, you must take sqrt(S21*S12). When you take the sqrt, you must guess at the sign, + or -. The delay tells us what to guess.