Microstrip TEM Mode
Good points but several things are mixed. I try to make an executive summary since I know a bit about this topic:
* The microstrip does NOT support a pure TEM mode at any frequency, due to the difference in propagation velocities between the two materials. cf. Pozar.
* It is not a matter of filling factor or material resistivity.
* The key parameter is the difference of speed between the two materials.
* This quasi-TEM mode is close enough to a TEM mode so it can be approximated by a TEM mode, and all relevant concepts of TEM modes (voltages and currents) can be applied without trouble. Say otherwise, there are not so much difference between the different ways of defining the voltage. Hence the name quasi-TEM.
* Particularly, for low enough frequencies, this quasi-TEM mode can be calculated in the same way as TEM modes.
* But this quasi-TEM mode gradually changes when frequency is increased.
* This dispersion needs to calculate the still quasi-TEM mode using full wave mode solvers.
But don't worry too much. There are lots more of real problems to care about.