Middlebrook's EET (Extra Element Theorem) Without Tears

Short and sweet.

Want a mnemonic? "The Prisoner of Zenda" - n before d basically. How about whether it's Z_n/Z or Z/Z_n? Just think what you need to get when you put Z = infinity and you're done.. Easy? See... (BTW, I strongly suspected Middlebrook had an English streak to him..) In his words :

• What's Z_d : It's the driving point impedance looking into the port where you're going to connect Z, with the input port shorted.
• What's Z_n : It's the impedance looking into the the port where you're going to connect Z, with the output port nulled for any value of the input. No matter what the input port voltage is, the output port voltage is 0 - which is not the same as shorting it to ground.

What about an input impedance? In that case, you might be pulling your hair out for Zn. If you trust the pros (Dr. V) just calculate the Z looking in with the test (port where Zin is determined) port open and you're done.. For a Zin function, the input and output ports are the same. What's the logic though?