Note:

In the above formula Z11 should be replaced by
Z11 / Z0. In other words Z11 is "normalized" with respect to Z0 in
this
formula. See the Gonzalez reference.

In a "normalized" Smith chart Z0=1.0.

Bill W0IYH

If the output is terminated with a 50 ohm pure resistance load then S11 is
the complex input reflection coefficient, 0 to 1.0. In an active circuit
(amplifier or oscillator) S11 can be outside these limits due to positive
internal DUT feedback.

Bill W0IYH