Re: transmission line z

On Tue, 10 Jun 2025 13:33:57 -0000 (UTC), "Don" <g@crcomp.net> wrote:

JM wrote:

john larkin wrote:
>

Suppose you have a slab of FR4 with copper on both sides, standard
ebay stuff. Now shear off a long thin slice. That's a balanced
transmission line.
>
air
__________________ copper
.........................................fr4
__________________ copper
>
air
>
>
>
What's that called?
>
Does anyone know of a calculator that handles this case?

>
It's just a parallel plate waveguide.
>
For the TEM mode Z0 = 377*sqrt(ur/er)*(d/w).
>
ur/er - substrate permeability/permittivity (relative)
d - dist between copper
w - width copper strip
>
Formulas for the higher modes also exist.
>
But I'd have to look them up.

>
Your free space impedance is easier for me to comprehend than the one
contained in Chemandy's calculator:
>
<https://chemandy.com/calculators/microstrip-transmission-line-calculator.htm>
>
Danke,

And that one assumes an infinite ground plane.

I suspect that all such formulas are wrong, except in a few rare cases
like a coax. EM simulation is better.