S-parameters simulation and characteristic impedance simulation issue

Hi,

for a physics calorimeter design project (for which the calorimeter could be made partly with pieces of PCB), I started some very simple PCB simulations with SIGRITY in order to get the S-parameters.

I started with a simple 100 ohms microstrip line on a 2 sides PCB (1 for the line, 1 for the ground place) with 2 SMA connectors for connection with external SMA cables.

the 2 ports on the SIGRITY simulation are the 2 SMA connectors.

It is possible to have a formula between the characteristic impedance and S11 and S21: Zc = R0 * sqrt((1+S11)^2-S21^2)/(1-S11)^2-S21^2))

If I apply this formula to the simulation (with a circuit simulator) of a simple 100ohms lossless line, I'm able to find exactly 100 ohms at all the frequencies.

If I apply this formula to the simulation (with a circuit simulator) of a 100ohms line surrounded by 2 short 50 ohms lines (which is equivalent to the 100ohms PCB line surrounded by 2 SMA connectors), I'm able to find 100 ohms at low frequencies and I get some peaks due to the impedance mismatch at frequencies multiple of round-trip frequency.

my problem is now when I do the same simulation with SIGRITY on the PCB microstrip line describe above, I don't get the same kind of result as the last circuit simulation.

below is the result of the simulation by SIGRITY with the "transmission line RLGC => characteristic impedance" parameter plotted.
the plot exhibit a 100 ohms characteristic impedance value (normally perturbed at high frequencies by the impedance mismatch with SMA connectors), but only after around 1MHz and a much higher value at lower frequencies.

does someone have an explanation or is it a bug?

thank you for any comment or help.

best regards

Ronic

PCB line 3D view