Re: transmission line z

On Tue, 10 Jun 2025 07:51:38 -0700, john larkin <jl@glen--canyon.com>
wrote:

On Tue, 10 Jun 2025 07:57:03 +0100, JM
<sunaecoNoChoppedPork@gmail.com> wrote:
>

On Sun, 08 Jun 2025 07:58:14 -0700, john larkin <jl@glen--canyon.com>
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.

>
That assumes that all the capacitance is confined to the rectangle
between the plates. Actually, that's good enough for what I'm doing
now, just making a txline transformer.

It's good enough for typical impedances used in PCB's (120 ohm or
less) where w/d > 1.

What Z0 do you need.