I've calculated the characteristic impedance of a CoPlanar Wave Guide (CPWG) type transmission line on my 8-layer stack-up between L1 - top layer and L2 - GND1 directly underneath it. The calculations are pretty standard and supported by many freely available calculators. The calculator by Spok consulting, for example, gave the following result:
I would like to increase the width of my traces whilst maintaining the same ~50 Ohm nominal characteristic impedance of my line. An obvious solution would be to have my reference plane further away from the top layer. Let's say I want to use L7 - GND. Naturally, I'd clear out all copper from between these two layers.
How do I calculate the effective permittivity of all the different layers of dielectric combined? I couldn't find any calculators that support this.