Using ADS 2016, how do I build a tri-metal microstrip ceramic substrate for EM modeling of skin depth with these three dis-similar conductivity mayterials?
I think the best option is to use one conductor only with equivalent metal parameters. These equivalent metal parameters can be verified by measurement, or fitted from an EM simulation that can handle stacked conductors properly.
Stacking multiple conductor materials will be difficult to model with Momentum, because Momentum simulates the conductors as hollow boxes. For each box surface, the frequency dependent surface impedance is calculated from material parameters, frequency and geometry. For your stacked conductors, you would then have two flat conductors at the same z position (bottom of upper conductor and top of lower conductor) and Momentum will apply precende rule to make one of them "win" and ignore the other. So in any case, directly stacking the metals with no gap will NOT give correct results.
Long ago, I once tried this stacked conductor materials with another MoM solver for a segment of straight line, and inserted a very thin artificial layer of air between the conductor layers, to avoid that "one metal wins and the other metal is ignored" issue. Then, you get correct results at DC, but have introduced artificial isolation between the sheets. What does that do? I don't know.
In my opinion, the only accurate way besides measurement: You can use the ADS FEM solver, but then need to make sure to mesh with "filled" conductors and discretize into skin effect. The ADS FEM default setting is to model conductors as hollow boxes with surface impedance, just like Momentum.
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