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I now get consistent values when using modal terminal ports or lumped terminal ports ! the way I was initialy configuring lumped port for differential pairs was not well done... This is now one problem solved  !

From my previous tests under HFSS, to get coherent values between Polar (polar do not take into account the bulk conductivity, only the delectric constant) and HFSS for a single ended caracteristic impedance, I needed to set bulk conductivity of aluminium to 0. So I have made some tests using both bulk conductivity to 0 or not for aluminium (I also made a test using the option "solve inside" when bulk conductivity is not 0) :

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When bulk conductivity of aluminium is not 0, I got a Zdiff between Z0 and 2xZ0, like [USER=134022]@D.A.(Tony)Stewart[/USER] said. But I do not get a Z0 value much less 45 ohms... I got 50 ohms. And I got Zdiff = 70 ohms. Ratio Zdiff/Z0 = 1,4 (when bulk conductivity is not 0 and "Solve Inside" option is enable, I got same values)

When bulk conductivity for aluminium is 0, I got Z0=88 ohms, Zdiff= 115 ohms and Zdiff/Z0= 1,3.

So it seems to get Zdiff around 100 ohms I need to set bulk conductivity to 0...

All I have to do now is test the actual cable to see what I get... but I don't know if with my nanoVNA I can measure Z0 and also Zdiff... I don't now if I can sent a differential signal on the two ports... But I hope it will give me some guidance ...