Re: transmission line z

On Tue, 10 Jun 2025 20:54:38 +0100, JM
<sunaecoNoChoppedPork@gmail.com> wrote:

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.

I need to put a fast kilovolt pulse into a 50 ohm load. I can use a
GaN fet and a transmission line step-up/isolation transformer.

I'd like to make the windings from PCBs with roughly 50 ohm
differential impedances, but if the txline windings are short compared
to rise time, it doesn't matter much.

I was just interested that this geometry is not included in any pcb
impedance programs that I know of. The imaginary equipotential plane
is a workaround.