# Discussion Forum

## Characteristic impedance or Lumped port impedance

Hello,

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.

Mari

4 Replies Last Post 2018年12月12日 GMT-5 下午4:38

Posted: 7 months ago

Mari,

your interpretation is exactly correct. You can also put it like this:

• Characteristic port impedance is the impedance of the external source that is connected to the port
• emw.Zport_n is the impedance of the model at the location of the port.

This allows to calculate S-parameters with regard to a certain reference impedance.

Cheers Edgar

-------------------
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com

Posted: 7 months ago

Thank you Edgar.

Posted: 5 months ago

Dear Sir Edgar, How to compute the S11 parameter in coaxial cable? The calculation of S parameter did not plotted in the graph of S parameter against frequency.

Posted: 5 months ago

There is no expression in the global plot.

-------------------
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com