complex impedance cst

Hi everybody, now I am studying the lossy case of Line impedance

First I create a coaxial line with inner PEC radius=1 mm and outer PEC radius=3.83 mm
the material between the two PEC is vacum, that is epsilon=1 mu=1, no conductivity.
and the line impedance is exactly the same as the analytical result.

But when I set the conductivity of the material between the inner PEC cylinder and a outer PEC shell to a non zero value, say, 0.1s/m, the line impedance shown on the port mode screen will still be a real number. The analytical result must be a complex number.

I can only find the value of line impedance when I show the port mode.

So my question is can CST show the complex Line Impedance, both the real part and the imaginary part?

By the way , I also create a same model in HFSS, and it is able to give a curve of both real part and imagninary part that are in accordance with the analytical result.

Regards!

Hi arthury,

in order to take into account losses for the port mode calculation you should use the FD (preferably the FD TET) solver. The Time domain solver does not take into account the losses during the 2D Port calculation. However, it does take them into account in the full 3D simulation.

If you use the FD TET solver you get the complex propagation const. as well as the -40dB distance. Still, the program only gives you the real part of the impedance. Looks like that the complex impedance is not extracted automatically from the 2D calculation. I have never missed this feature but if you do I would simply contact the CST guys. Maybe they have a results template which extracts the complex impedance. Should be possible since all important data (fields!) are already calculated.

F.

I really appreciate your kindness.

Yes, I used to do simulation under CST, and is quite familiar with it.
but this time it is so strange that such a good software will not give img part of Z0

Now I am forced to use HFSS, but I really hope to use CST to solve this problem because I am more familar with FDTD rather than FEM

Hi arthury,

as I said: I would contact the support guys. Maybe they have a post processing template to extract the line impedance out of the port fields.

F.

Hello arthury !

If you have MWS 2006 with MW Design Enviorment you should add an external port and calculate the circuit (which is taken from the MWS enviorment) and inspect
the results of Impedances.
There you can see the Imaginary and as well the Amplitude magnitude of the |Z|.

Good Luck !

Itzik.

Hi itzikhaim,

this would be the complex impedance of the full structure. I agree you could directly plot them in MWS as well by performing a Y/Z calculation from the S-Parameter.

arthury is interested in the complex port impedance (e.g. 50 Ohm - j 0.03 Ohm). This is something different...

F.

Hello RFSimluator !

You were right I didn't notice the full question.
The fact the CST shows the real impedance instead the complex, its because they
calculates the Zo with help of the power and the currents related to the port (physically).

To be more accurate:
Z = Power / (Sum[current])^2
Power is given by integral of poynting vector over the port area.
Currents calculated by integrating magnetic field in small distance around the
conductor's surfaces.

This is true for TEM (or QTEM) modes.

Itzik.

HI,
I consider ur question and try with my case
U can calculate the real and image part of the line impedance and referenced (normalized) impedance by a easy way: Resuls-->S -parameter calculation-->calculate Z and Y matrix!
I think these are real and image part of our line impedance. Right?
Try and share ur experiences!

Hi KhangKhang

nope.. This would be the complex feed point impedance for the full structure. (Or better the Elements of the impedance and Admittance Matrices). Don't mix this one up with the Complex Line impedance. ...

The Elements of the Impedance and Admittance Matrices tell you how the ports of the structures interact which each other thru the structure. (Pozar pp 191)

The complex line impedance is a purely "local" value which is only valid for a particular part of the line. This value does not care how the full structure looks like and if you have any other port.. In most books only the "real part" of the line impedance is considered.

F.

P.S. arthury: do you have any good reference for the complex line impedance for typical transmission lines. I'm looking for some analytical and/or empirical formulas. All the books and impedance calculators I have only give me the real part...

Thx.

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