David didn't answer your question, of course impedance
is a complex value. But the question was is the
formula he posted the general expression for Input
impedance from S11 and the answer is yes. Do remember
to convert the S11 from dB to linear terms, where
it is a voltage ratio. This can be used to convert
the S11 measurement of the Zo =50 or 75 ohms of
the measurement instrument plane to the Zload of
GammaL zero occurs when the load
impedance is equal to the reference impedance i.e.
when both impedances are exactly matched.
Sunshine Design Engineering