Characteristic impedance or Lumped port impedance
Posted 2018年10月11日 GMT-4 上午8:20 Electrical, AC/DC & Electromagnetics, RF & Microwave Engineering Version 5.3a 4 Replies
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There are some parameters in Comsol RF module which are unclear to me.
To understand this question I created a very simple example: - coaxial cable of 50 Ohm impedance, perfect electric conductor, L=0.75m; - lumped port 1 with wave excitation; - lumped port 2 terminating the line at the end, - Electromagnetic Waves, Frequency Domain.
Characteristic impedance is a value specified for a lumped port. Lumped port impedance is emw.Zport_1 calculated in Results -> Derived values -> Global evaluation .
So the question is, what those two values mean?
I expected the Lumped port impedance to be the impedance of the lumped port, but apparently it is not :D
Example 1: f0 = 100MHz, Z1 = 50 Ohm (Characteristic impedance for the port 1), Z2 = 50 Ohm (Characteristic impedance for the port 2). Results: S11 = -3.5e-4+8e-6i - zero reflection (matched case), emw.Zport_1 = 49.979+6e-4i - equal to Z1.
Example 2: f0 = 100MHz, Z1 = 25 Ohm (Characteristic impedance for the port 1), Z2 = 50 Ohm (Characteristic impedance for the port 2). Results: S11 = 0.33+7e-6i - finite reflection for unmatched case, corresponding to , emw.Zport_1 = 49.979+6e-4i - not equal to Z1, but equal to the coaxial impedance and port 2.
It seems that Lumped port impedance (emw.Zport_1) is not the impedance of the port itself, but impedance of all elements together which are after the port. It seems also that Characteristic impedance is indeed the impedance of the port, even though Comsol provides a confusing name for it.
If someone knows exactly the definitions of the discussed parameters, please help me with this question.