## Microstrip TEM Mode

• 0
According to https://www.microwaves101.com/encyclopedias/microstrip, "Microstrip does not support a TEM mode, because of its filling factor."

However, this article says that it works if the spacing to the dialectric is much smaller than the wavelength:
With microstrip, a portion of the electric fields are in the dielectric between the strip and the ground plane while other fields exist in the region above the strip with air as a dielectric. At frequencies where the electrical distance in the dielectric material between the strip and ground plane is much less than a wavelength, microstrip behaves as a non-dispersive TEM line.Unlike moding in pure TEM lines, the transition in microstrip from TEM to quasi-TEM is not sudden. With increasing frequency, as the substrate thickness of microstrip becomes appreciable, the propagation velocity and the characteristic impedance of the line increase.

Does the 101 microstrip page need updating to indicate that?

-Eric
• 4
• 0
Yeah i think you are right. It need an update to clarify that.
• 4
I update the that page slightly, how does this look?

https://www.microwaves101.com/encyclopedias/microstrip#types

Steve
• 1
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.
• 0
I have a related question... In quasi TEM transmission, waves in the two dielectrics try to travel at different velocities, does this mean that if the TL is long enough, you can have opposite phase in the fields within one dielectric and the other? If so, does this have impedance implications? (e.g. will the impedance be different at different points in the TL?)
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That is too hard of a question for me! Can you simulate it in HFSS? Also, if there was a way to make "air microstrip" I think we could call it TEM at last. Maybe we could use magnetic levitation to hold up the conductor.

Steve
• 0
LOL... you could just make an air stripline and remove the top cover!
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