From: Don Pearce
Subject: Re: How to check for a 50 or 75 ohm connector
Date: Sun, 22 Dec 2002 15:31:56 +0000
References: <3E053579.AFE0BFDF@sympatico.ca> <3keudKjAZXB+Ewcs@newbrain.demon.co.uk>
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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.