HFSS layered impedance (multiple thin layers) effect on cavity Q

[nevetts]

Can anyone tell me the best way to simulate mutliple thin layers in HFSS. I'm currently having problems with the layered impedance boundary...

I'm simulating the effect of a thin layer of Nickel Chromium underneath a thin layer of gold on the wall of a rectangular cavity operating at the fundamental mode (TE101). To impose this boundary condition I use the layered impedance boundary.

If the thickness of the gold is less than the skin depth I was expecting the cavity to become very lossy and for the Q to become much smaller. However when I set the thickness of the gold to be many skin depths I expect the cavity Q not to depend at all on how much nichrom is behind it. Unfortunately the HFSS result shows that, even for 1cm of gold, the cavity Q begins to drop for about 100 microns of nichrom.

Ideally I'd like to be able to trust the results for about 10nm of gold and 3 microns of nichrom but since the limiting case (lots of gold) is so clearly wrong I'm not sure I set the simulation up right.

Did I use the layered impedance boundary wrong???

The HFSS file is uploaded. Any help is appreciated!

 

[mkonca]

Firstly, thanks for asking a good question. I like this forum but I am so fed up with people asking things such as "teach me how to build the best antenna in the world! NOW!".

As for the answer: I don't really know. I use HFSS on a daily basis but never needed the layered impedance boundary.
From what I see, it is just an impedance boundary but it automates the process for you: When you put in the layers in the dialog box and hit calculate it assigns that calculated impedance to the surface. So try different values and hit calculate and see if you are getting an impedance in the expected range.

Also I have a question, what if you just defined the surface in 3d? I know it would take more resources but may be you could compare a small 3d sample to the impedance surface and see if the results are the same.

Good luck, I am interested in the solution to this.

 

[nevetts]

Thanks for the response!!

Actually the first simulation I did (before learning about layered impedance) was a 3D simulation. This seems to fix the problem of 100 um of nichrome making a difference under 1cm of gold. But it has a different problem.

When the gold gets thin enough the Q just keeps getting lower and lower (as low as 10). I was expecting it to bottom out at at about 3000 which is what I get if I take the gold out of the simulation altogether and just use a nichrom wall...Maybe I set up the 3D model wrong? (see attachment)

Additionally, according to your other suggestion I looked at the impedance that HFSS calculates with the layered impedance boundary. It still shows a dependence on the nichrom underneath 1cm of gold!