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Problem with Mesh of Stripline Ground

Question asked by SDFGSDFGSDF on Jul 28, 2006
Latest reply on Nov 23, 2006 by SDFGSDFGSDF

I am trying to investigate the effects of cutouts in the top side ground of a stripline using Momentum. The layer stack used is:

open boundary,
top ground (metal strip),
signal conductor (metal strip),
closed boundary (perfect conductor).

The reason for using explicit metal strip and not closed boundary for the top ground is to be able to draw holes or slots into the ground, which are to be investigated.

If I fill up the top ground (no cutouts), I expect that the resulting (ideal) stripline has parameters as predicted by LineCalc. However, looking at results from Momentum simulation, the characteristic impedance is too high, i.e. 60 Ohms instead of 50 Ohms. (In contrast, if I simulate the same transmission line but with the top ground modeled by a closed boundary, the line parameters obtained from Momentum simulation agree very well with those from LineCalc.)

I found that when increasing the mesh density to beyond 45 cells per wavelength, the line parameters gained from Momentum approach those obtained from LineCalc. I think that the problem lies in the fact that with a coarse mesh, the current density in the top ground plane (metal strip) is not approximated correctly. Going to finer mesh increases the transversal resolution of current density, giving more accurate impedance information. (Simulation settings: Edge mesh 0 um active, thin layer overlap extraction and mesh reduction are also active, always in microwave (not in RF) mode.)

As I read the help to the mesh setup, a feature called thin layer overlap extraction has been introduced to prevent the problem I mentioned above. However, whether it is enabled or not, the cells of the mesh in the top ground are much wider than the width of the signal conductor.

My main question: How can I force Momentum to generate a denser mesh in the top ground in the vicinity where it overlaps with the signal conductor?

An alternative would be to map the top ground layer as metal slot (instead of strip). What could be expected from this approach?

Any help on this is greatly appreciated.