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From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: transmission line z
Date: Tue, 10 Jun 2025 07:51:38 -0700
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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.