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I have two sets of S11 data (S1P format) which are measured on an open co-axial cable (~ 20 cm long) and open-end as reference (see the hand-draw figures below) correspondingly. I used the Python skrf package code to do TDR on these data. The main code looks like this:

# Add DC component to the S11 data
sample = data.s11.extrapolate_to_dc(kind='linear')
# Find the step response through impedance vs. time
sample.s11.plot_z_time_step()

In the open-end case, I'm expecting a high impedance at t=0, which means the signal is totally reflected at the end of the cable (where the VNA is calibrated), and it does.

In the 20 cm cable case, I'm expecting a high impedance at a delayed time after t = 0, but it seems that it occurs at t = -1 ns, which means that the reflection occurs earlier than the free open-end. I suppose the curves are correct, but the origin of time is somehow shifted. Can anyone give me some hints on how the time axis is determined during the step response calculation?

enter image description here

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