waveguide cst eigenmode

Hallo,
when doing eigenmode analysis in CST Studio on a microstrip configuration,
I get different results for different line lenghts.
First of all, does cut-off frequency mean the lowest frequency on which the
specific mode is able to propagate ? I think so.
Well, if my lines are short enough, CST finds two modes (HE0,=QTEM, HE1) with cut off frequency 0 Hz, and some higher ones. This is right.
But with longer lines, the AKS solver calculates cut off frequencys greater than 10 GHz.
Any ideas ?
Thanks

Hi elektro0,
I'd suggest that you check again your boundary conditions in both of the models.
Can you post the model here?
Regards,
P.

I also have the same question, Can anyone tell me.

What about "cut off" frequency ? Is it the lowest possible frequency for the mode ?
Wont anybody confirm it ?
@Pushhead.
Thanks for your answer.
I am using conducting wall (copper) boundaries. My model consists of a microstrip line (microstrip over ground plane), which is connected through a via with another microstrip line.
I expect two modes with cut off frequency 0 Hz.
The QTEM=HE0 itself and the higher mode TM0 (surface wave).
According to "Meinke, Grundlach", there should be higher order modes
HE1, TE1,....
I have waveguide ports in the model. I think it will be ignored by the eigenmode solvers AKS and JDM. Right ?
If I run an eigenmode calculation with my normal setting, I only get waves with cut off frequencys higher than 10 or even 20 GHz.
This is definitively nonsense.
Simply reducing the line length of the microstriplines makes the two modes with cut off 0Hz possible, when doing eigenmode analysis.
So, it shouldn't have something to do with the boundary setting.
Actually I dont know if I can trust the results. I will need an argument for it.
Thanks for any reply.



I already use

CST eigenmode solver admits only closed structures, so probably your stripline model is not accurately represented, unless it is enclosed into a metalic box.
regards,
mpero

ok.
@mpero.
The only alternative to me, is taking magnetic boundary, at least at this areas, where the strip ends. I tried to do it, with the periodic setting.