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For a 2-port structure, there is no Zin and Zout clearly defined. Any possible Zin or Zout needs to be under some conditions. For example, you can define Zin is the input impedance at port 1 when port 2 is open-circuited, and Zout is the input impedance at port 2 when port 1 is open-circuited. If you define them in such a way, Zin = Z11 and Zout = Z22. However, you still need some other independent conditions to find out Z12 and Z21. You need 3 independent measurements in order to solve Z11, Z22 and Z12. If Z21 is not the same as Z12, you will need 4 independent measurements to solve the 4 quantities.