Horror Vacui
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S11 is like a reflection coefficient. There is a bijective connection between reflection coefficient and normalized impedance. No conversion is needed on the smith chart - that's point of the chart! - just different scales are used.
S21 is a ratio of incident and reflect waves. The waves are merely on different ports, so in princip it is the same as S11. Nevertheless I do not see the point of using a Smith chart for S21. The middle point of the Smith chart has no meaning here, no matching is required in this parameter. If you have S21=10+j0, then where would you add it on the Smith chart? A conventional magnitude plot says more about S21.
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