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From: Cursitor Doom <cd@notformail.com>
Newsgroups: sci.electronics.design,sci.electronics.repair
Subject: Re: Francophones
Date: Wed, 25 Dec 2024 14:48:20 +0000
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On Tue, 24 Dec 2024 22:42:17 +0000, Ian Jackson
<ianREMOVETHISjackson@g3ohx.co.uk> wrote:

>In message <6ftjmjppf4421dl2ec0ek4mvfht74lmnu2@4ax.com>, Cursitor Doom 
><cd@notformail.com> writes
>>On Mon, 23 Dec 2024 23:05:03 +0000, Ian Jackson
>><ianREMOVETHISjackson@g3ohx.co.uk> wrote:
>>
>>>The real difference between the two impedances is that the amount of
>>>PTFE dielectric in the 75 has been minimised in order to increase (with
>>>some difficulty) the Zo from 50 to 75 ohms. IIRC, the 50 has a
>>>more-uniform structural RLR, so it the better connector at the higher
>>>frequencies.
>>
>>I don't think that's quite right. The diameter of the inner and outer
>>conductors has more influence on Zo than the dilectric thickness.
>
>It's right all right.
>
>The outer diameter is the same for the 50 and 75 ohms. For the 75, I 
>presume it's not practicable to make the pin diameter smaller and retain 
>its robustness, so the only way to increase the Zo is to remove as much 
>of the dielectric as possible. If you compare the 50 and the 75, you 
>will see what I mean.

I still maintain the principal determinants of the impedance are  as I
stated previously. The formulas for line impedance are shown on this
page and the aforementioned determinants are key.

https://www.everythingrf.com/rf-calculators/coaxial-cable-calculator