Re: 50 ohm on FR4

On 2/2/2025 12:42 PM, Cursitor Doom wrote:

On Sun, 2 Feb 2025 12:00:24 -0500, "Edward Rawde"
<invalid@invalid.invalid> wrote:
 

"Cursitor Doom" <cd@notformail.com> wrote in message news:pssupj9omf6d3ao3jpilfjmhkommdf0m6e@4ax.com...

Grok reckons to make a 50 ohm tx line on FR4 I would need  a Trace
Width of approximately 2.73 mm and a Trace Spacing of at least 2.73
mm. Does that sound about right?

>
It doesn't seem to match anything I can get from the site below with 2oz copper and er=4
Does Grok know the difference between stripline and microstrip?
>
https://www.elektroda.com/calculators/pcb-impedance-calculator-microstrip
>

 Thank you, Edward. Well, to be fair to Grok, it did spell out a lot of
qualifiers relating to the precise structure of the FR4 substrate
which I did not include in the OP. So that could well have something
to do with it. Grok also felt that performing such calcs was somewhat
out of its comfort zone and referred me to specific websites for these
calculations. However, I just wanted a quick and dirty figure as I'm
not attempting metrology-level measurements in this instance and will
not be venturing beyond VHF.

The pen-and-paper method given in Pozar's "Microwave Engineering" is to guesstimate whether W/d is >= or <= 2, use the second part of equation 3.197 to find A and B, use the first part to see if the guess is satisfied, use 3.195 to find the effective dielectric constant, and verify with 3.196.
<https://imgur.com/a/Dr23fNi>
Or use the calculator included in KiCad..I get more like 3.1 mm for a width with the parameters given
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