When I use the Port Extensions in PNA, I want to know how to get the DC loss term in the port extension setup ? In Dr.Joel's book ,Chapter 9,equatin 9.23 and 9.24 don't have this DC loss term, if the term is added into the equations, how to use the the least squares fitting over the current span of the measurement in chapter 9.4?

DC offset term is needed for the case where there is some fixed attenuation in the path, in addition to cable loss. For example, if there were a 3 dB pad and a cable. The PNA applies the loss vs. freq equation and then adds the DC offset as a fixed offset.

In APE, the loss term is extracted as the Y-axis intercept.