Hi Alan,

There are 50 and 75 ohm versions. For most applications, it doesn't
matter much which you use since the path length through the connector
is short compared to a wavelength at typical frequencies.

Note that the impedance mentioned here is characteristic impedance,
which is not the same as source or load impedance. Characteristic
impedance is the impedance seen when you send a signal into an
infinitely-long transmission line. For a line of finite length, this
impedance is what is seen by a pulse entering one end until the signal
has a change to travel down to the far end and reflect back, at which
point the input impedance changes to something which depends on what
the termination at the far end is. For a 1 inch long BNC connector,
this would only be about 200 picoseconds, so it is irrelevant for most
applications.

Another way to think about this is the following: when you terminate a
transmission line in the same impedance as its characteristic
impedance, then the impedance seen looking in the open end is
independent of the cable length. If you terminate it in something
else, then the impedance becomes a periodic function of the length of
the cable in wavelengths at the frequency you are using.

Sean


On Tue, Oct 14, 2008 at 10:13 AM, alan smith <micro_eng2@yahoo.com> wrote:
> I was asked this the other day and my first gut reaction was..no, shouldnt be.  For a BNC coupler, is there a 50 and 75 ohm version?  Or is it just a coupler so the impedance should be zero or close to it, since its coupling an impedance matched connector system.
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