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From: Jeroen Belleman <jeroen@nospam.please>
Newsgroups: sci.electronics.design,sci.electronics.repair
Subject: Re: Francophones
Date: Fri, 27 Dec 2024 16:23:58 +0100
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On 12/26/24 00:15, john larkin wrote:
> On Wed, 25 Dec 2024 14:48:20 +0000, Cursitor Doom <cd@notformail.com>
> wrote:
> 
>> 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
> 
> That calc is nonsense. Reasonable entries generate negative values and
> a preposterous cutoff frequency.
> 
Negative values for Z0? That would be surprising, because
the equation is correct, even though the physics are
obfuscated away into a few magic factors.

Example please?

Jeroen Belleman