hi all, good day to you.

i am using PNA to do some characteristic impedance measurement, and with my DUT connecting to the front panel input, assuming i set S11 measurement, in smith chart format, and i managed to get 50ohm result, which indicate good absorption power of the DUT.

However, i feel abit confuse that can i claim Z11 is the input impedance characteristic? and from below documents:

http://contact.tm.agilent.com/data/stat ... n-95-1.pdf

i find out we have two different formula, on the page 13 we have Z1 = [(1 + S11)/ (1-S11) ] * Zo

while on the page 65 i have

Z11 = (1+S11)(1-S22)+ S12*S21 / (1-S11)(1-S22)+ S12*S21

which when i apply this formula, it is not identical to the result i measure in the smith chart but Z1 seems working fine.

Can anyone help on this issue?

thanks for your help.

i am using PNA to do some characteristic impedance measurement, and with my DUT connecting to the front panel input, assuming i set S11 measurement, in smith chart format, and i managed to get 50ohm result, which indicate good absorption power of the DUT.

However, i feel abit confuse that can i claim Z11 is the input impedance characteristic? and from below documents:

http://contact.tm.agilent.com/data/stat ... n-95-1.pdf

i find out we have two different formula, on the page 13 we have Z1 = [(1 + S11)/ (1-S11) ] * Zo

while on the page 65 i have

Z11 = (1+S11)(1-S22)+ S12*S21 / (1-S11)(1-S22)+ S12*S21

which when i apply this formula, it is not identical to the result i measure in the smith chart but Z1 seems working fine.

Can anyone help on this issue?

thanks for your help.

Z11 is the first of 4 impedance parameters describing an invariant behavior of your network, and does NOT change with loading of the other port. The devinition of Z11 is V1/I1 provided V2 is open. But, you are loading it with 50 ohms, nominally, and you don't measure V1 and I1 you measure a1 and b1 when you measure S11, which is defined with port 2 terminated in 50 ohms. So the Z11 computation gives a different answer.

To understand Z11 you must see that it is defined as V1=Z11*I1+Z12*I2; and V2=Z21*I1*Z22*I2, so when you load the port 2 with 50 ohms, I2 is no longer zero, so it will not be the same as Z1.