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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 19:48:27 +0100
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On 12/27/24 17:21, john larkin wrote:
> On Fri, 27 Dec 2024 16:23:58 +0100, Jeroen Belleman
> <jeroen@nospam.please> wrote:
> 
>> 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
> 
> https://www.dropbox.com/scl/fi/aglpygrtir3m9bccil1to/EvRfZcalc.jpg?rlkey=nazlwgbmpvbssz5zv2a92hrqj&raw=1
> 

Indeed! They seem to have botched the inch and cm units.
It makes better sense for the other units. It's a weird
and wonderful error to make, because it doesn't actually
matter in which units the diameters are given, as long as
they are the same! The argument of the log is dimensionless!

Shame! Obfuscate the physics and then get it wrong too!

Jeroen Belleman