I have a model that I want to know its impedance. The model is excited by lumped port (0.5V incident voltage which equals to 1V of source in experiment ). the characteristic of port is 50ohm. I plot S11 and found out that the most voltage is reflected as it was around -0.1dB or less. I draw a line on the lumped port (the rectangular that connect two conductors on top and bottom) in order to calculate input voltage, but as I do not know the incident current I cannot measure impedance by Z=V/I. what should I do? is the Z-parameter plot the impedance of load or not? I plot the Z11, but I am not sure that is it load impedance (HFSS model) or not?
Hi, Najme Mohammadyahya
Yes, the Z11 calculated from the measured S11 is the input impedance of the HFSS model (for a one-port network).
- NO! Z11 is NOT an input impedance of the model. Just open any book on RF engineering. Z11 may approximate the load impedance only for antennas loaded with high enough impedance. i.e. when 1/ZL->0
In other cases, you need to derive the input impedance knowing the full Z matrix, and the full set of loads.
The simple case of 2 port networks can be found... just ANYWHERE. Like in Wikipedia
https://en.wikipedia.org/wiki/Impedance_parameters
Zin=Z11-Z12*Z21/(Z22+ZL).
For 3-pot network it is derived from 4-port network equation, where you substitute every Z in above formula with rank-2 matrices (like splitting every port in two. See T. Reveyrand "Multiport conversions between S, Z, Y, h, ABCD, and T parameters"). Then you shunt one port of 4, and get:
Zin1=Z11-((Z13*Z31+Z12*Z21)*ZL+Z12*Z21*Z33-Z13*Z21*Z32+(Z13*Z22-Z12*Z23)*Z31)/( ZL^2+(Z33+Z22)*ZL+Z22*Z33-Z23*Z32 )
for equivalent loads, i.e all connecting lines are ZL=50 Ohm... if not, it is a bit more complicated.
Now, the question is HOW you derive the load impedance... I think, we discussed this with you in another topic.
If you know the source impedance - which you have set at 50 ohms - and the value of S11, you can easily calculate the load impedance.
You need to use the complex value of S11, and you get a complex impedance.
Z=50*(1+S11)/(1-S11).
The normalised load impedance (1+S11)/(1-S11) can be read directly off the Smith Chart impedance display.
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