Re: transmission line z

On 6/8/2025 3:34 PM, john larkin wrote:

On Sun, 8 Jun 2025 15:19:18 -0400, bitrex <user@example.net> wrote:
 

On 6/8/2025 1:01 PM, john larkin wrote:

On Sun, 8 Jun 2025 11:41:19 -0400, bitrex <user@example.net> wrote:
>

On 6/8/2025 11:39 AM, Bill Sloman wrote:

On 9/06/2025 12:58 am, 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?

>
Neither of my transmission line textbooks say anything about that. It's
clearly a kind of transmission line, but presumably not a useful one -
it's going to be dispersive, just like microstrip. If it was useful you
would presumably be able to buy reels of it.
>

>
Used that way it's called a "parallel plate line" and the essential
characteristics for ye olde Telegrapher's Equation are given in Pozar:
>
<https://imgur.com/a/gAbKJgk>

>
Except that between the plates is FR4, dielectric constant near 4.

>
It's okay, in those equations epsilon prime is defined as e_r*e_0, and
then epsilon double prime is related to the loss tangent of the
dielectric is related as tan(delta) = (omega*e'' + sigma)/omega*e'

 But all the capacitance is assumed to be confined into the central
rectangle, with the rest of the universe having e=0.
 That's maybe good enough if the dielectric constant of the stuff
between the plates is high, and the w/t ratio is large. But it ain't
right.
 

Simple solutions are for simple problems, and all that...
If the frequency is high enough you can use that structure as a waveguide, too! It supports TE, TM, and TEM, though some modes may be more appropriate than others, depending