thank you andrew for your response.
actually, the results i got, show that the input impedance is a function of the source impedance (the impedance of the port), so there is to possible response:
the psp is not the good analisys i have to run,
or the input impedance do really change with the source resistance ( it's the first time i'm seeing someting like that)
so my question was to know wich is the good response to my problem.