A line length estimate (electrical delay or free space equivalent length) can be used to help with root choice, but this is not a critical parameter. Typically, one needs only to be within a half-wavelength of the correct length at the maximum desired calibration frequency. If one enters 0 for the length, the software will automatically estimate the length based on fitting-to-the-phase function at the lower part of the frequency range. Details are discussed in Adapter Removal Calibrations and Network Extraction related to automatic length estimation for adapter removal procedures and the same principles apply here. The only danger in auto length estimation is if the frequency step size is large relative to the electrical size of the setup. As discussed in Adapter Removal Calibrations and Network Extraction, one can look at the phase of an uncalibrated transmission parameter in the setup to see how fast the phase is changing.

If the match of the reciprocal network is worse than –8 dB or the loss exceeds ~20 dB, the reciprocal treatment will start to degrade, but a calibration will still be possible. Since such a network is at the limits of de-embedding capability, there are few choices except to consider 1p–2p processing with scalar de-embedding.