From: Ian Walker
Subject: Re: How to check for a 50 or 75 ohm connector
Date: Sun, 22 Dec 2002 15:07:35 +0000
NNTP-Posting-Date: Sun, 22 Dec 2002 16:00:56 +0000 (UTC)
User-Agent: Turnpike/6.02-U ()
In article <firstname.lastname@example.org>, DarkMatter
>On Sun, 22 Dec 2002 08:22:24 +0000, Ian Walker Gave
>>In article <3E053579.AFE0BFDF@sympatico.ca>, Dave Holford
>>>Chris Carlen wrote:
>>>> George Shaw wrote:
>>>> > I have a large box of mixed BNC connectors/tees etc.
>>>> > Using the simplest equipment possible (multimeter?) HOW do I check if
>>>> > the connector is for 50 Ohm or 75 Ohm coax connection. There are no
>>>> > external markings.
>>>> I find it very difficult to accept the suggestions that 50R vs. 75R
>>>> connectors have different diameter pins, as that would imply that the
>>>> "BNC" connector has different mechanical standards for different
>>>> impedances. If this is truly the case, then I will be eager to learn
>>>> that it is so.
>>>Although I have never noticed any obvious difference in the dimension of
>>>BNC connectors, other than the cable size, I have always understood that
>>>the difference between the 50 and 75 Ohm "N" connectors was the pin size
>>>and that attempting to interconnect them could result in damage to the
>>>connector if attempting to insert a large pin, I forget which was which,
>>>into a small diameter socket.
>>It is an urban myth that the centre pins of 50 and75 ohm BNC connectors
>>are different diameters, probably brought about by an optical illusion
>>caused by the taper of the pin along with the knowledge that the inner
>>to outer ratio must be greater for higher impedances. This overlooks
>>that using a different dielectric will change the impedance, i.e. more
>>air as in the case of 75 ohm BNC connectors.
> Well, it isn't "air". Coaxial cabling usually incorporates a
>polyvinyl foam or teflon or other insulator as the dielectric sheath
>over the center wire. ALL BNC pin diameters are the same. The
>difference is that connectors are made to fit their rear ends into
>different cable shapes. THAT is where they differ. The plug side is
>always the same.
Wrong, I use both 50 and 75 ohm BNC connectors, they are different;
although the diameters of the inner and outer conductors are the same
the amount of dielectric between them ,and therefore the amount of air,
> Most folks that work with them on a daily basis can tell by
>examination of the rear, cable insertion end of the device in
>question. RG-58, RG-59, RG-62, RG-6, RG-174... etc. ALL have BNC
>iterations, and all are different sizes. Hell, RG-174 is like an
>eighth inch in diameter.
I have seen N types made for RG-174 & 6, I find the cable side difficult
to judge since there are many different cables and looking at the outer
diameter as indicated by the rear of the connector tells you nothing
about the inner diameter or the dielectric of the cable it is made for.
Only looking at the front of the connector tells you its design