S para overturn,is there someone know why?

as in the picture , the S para's phase overturn at about 3.2GHz ,why does it overturn ,and sometimes they overturn at different freq ,does it mean to be something , plz help me !
thanks a lot!

I think, that -181° is the same as +179°. Its OK. What is the picture ? Is the simulated or mesured ?

it's measured curve , i just don't know why it overturn , the reason.thanks

This is not overturn. We need to understand that phase is not 360 degrees in practice. Based on the physical length of the DUT this phase characteristics will change.
If you add an adopter and repeat the mesurement then you will not see the overturn at the same frequency, let say at 3.2GHz.

Don't panic you are just seeing the aliasing of the phase because the VNA only shows -180 to 180 degrees.

Look at the discussion of wrapped vs un-wrapped phase on this site.

As others have said this is an artifact of the data/display being limited to +/-180 degrees.

You are using ADS to display the results so if you wish to see the data without the 'overturn' use the unwrap() measurement expression which will compensate for this behaviour. Place a Measurement Expression (MeasEqn) on the schematic or an Equation (Eqn) in the Data Display such that:

A = unwrap(phase(S22))

Then display A on the graph instead of phase(S(2,2)).

yeah,i got it,thanks for your answers all.