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I am trying to achieve impedance matching using load-pull mechanism and I recently found out that on a Smith chart in the VNA, the S21 doesn't make any sense as I have to plot transmission parameters on a polar plot.

But what if I achieve impedance matching (point at the 50 Ω impedance match), does that mean I really achieved impedance match even though the R+jX components are not 50+j0 or 1+j0? And why should I consider plotting s21 on a Smith chart?

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  • \$\begingroup\$ The S21 of what? \$\endgroup\$
    – hobbs
    Commented Jan 24, 2023 at 9:30
  • \$\begingroup\$ After TRM and TRL calibration, I have observed that S11 and S22 are matched(exactly at the centre of the Smith Chart) but the transmission parameters S21 and S12 are not. I have created a setup using waveguides and a magic T. The idea is that I take some metals or some solid materials of several different lengths and use them as sliding stubs to change the load. I have found out that the waveguides have an impedance of around 500 ohms. But when I am making both the planes match in the centre, and they do but I am not able to achieve the ideal impedance which 1+j0 or 50+j0. \$\endgroup\$
    – Viswanath Nimmagadda
    Commented Jan 25, 2023 at 11:31

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A Smith chart is simply a polar plot of the magnitude and phase of a number. It's useful to plot the reflection coefficient, S11 on a polar plot, and overlay a special scale in ohms.

After Thru calibration, S21 will appear to be 1.0 + j0.0. On the polar plot / Smith chart this is a spot at the left.
It would be unusual to plot S21 on a polar or smith chart as it's quite hard to interpret anything useful from it there.

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