From: Ian Walker
Subject: Re: How to check for a 50 or 75 ohm connector
Date: Sun, 22 Dec 2002 16:03:58 +0000
NNTP-Posting-Date: Sun, 22 Dec 2002 17:00:49 +0000 (UTC)
User-Agent: Turnpike/6.02-U ()
In article , Don Pearce
>On Sun, 22 Dec 2002 10:38:02 +0000, John Woodgate
>>I read in sci.electronics.design that Ian Walker
>>wrote (in <3keudKjAZXB+Ewcs@newbrain.demon.co.uk>) about 'How to check
>>for a 50 or 75 ohm connector', on Sun, 22 Dec 2002:
>>>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.
>>No urban myth. I have both sorts of connector, marked with their
>>impedances, and the centre pins ARE of different diameters. It MAY be
>>possible to use the same size pin and different insulators, but I don't
>>have any examples of that.
>Of course, now I think about it - the diameter of the pin doesn't
>actually do anything for the impedance of the connector when it is
>mated. It is the outer diameter of the female jack that forms the
>impedance-determining dimension in the connector.
But only in conjunction with the dielectric constant of the material
between the inner and outer contacts.