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From: john larkin <JL@gct.com>
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Subject: Re: Francophones
Date: Wed, 25 Dec 2024 13:49:04 -0800
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On Mon, 23 Dec 2024 23:05:03 +0000, Ian Jackson
<ianREMOVETHISjackson@g3ohx.co.uk> wrote:

>In message <nnd$23e91f0d$053bf96e@d25fd620e9918bf1>, Arie de Muijnck 
><noreply@ademu.nl> writes
>>On 2024-12-23 16:22, Ian Jackson wrote:
>>> For many purposes you can ignore the difference between the 50 and 75 
>>>ohm impedances (and 60 if you ever come across any). However, be very 
>>>aware only BNC 50 and 75 ohm connectors are mutually mechanically 
>>>mateable (yes - they really are!). I don't know if any others that are.
>>
>>Not recommended.
>>The center pin differs, a 50 Ohm BNC pin is thicker and may damage a 75 
>>ohm BNC.
>>I'm cautious, most of my below 1 GHz equipment is 75 Ohms (from CATV 
>>company), the rest is 50.
>>
>For most of its length, the male BNC 50 ohm pin is actually the same 
>diameter as the 75. However, the tip of the 50 pin is fairly 'blunt', 
>while the 75 is more pointed.
>
>I guess that if you are particularly clumsy while you are inserting a 50 
>male into a female 75, it might just be possible to have the blunter 
>male pin a bit off-centre, and catch the side of the female receptacle, 
>and splay it. However, despite 43 years working in the CATV industry, I 
>failed to achieve this!
>
>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.

The critical part of the connector here is a fraction of an inch long,
so none of this stuff matters below a few GHz.