> Justin Richards wrote:
> > Is there an implied frequency when discussing characteristic impedance.
> > When we need to analyze cables with respect to their RF
> > characteristics we end up with a large table S parameters that have
> > been recorded over a range of frequencies.
> >
> > So when a cable is quoted at 75 ohms, that must be for a given
> > frequency.  If so what is it.
> >
> An ideal coaxial cable (made up of perfect conductors and perfect dielect=
rics in a
> perfect geometery) will have a constant characteristic impedance across a=
ll
> frequencies. I'm not sure on the details but my understanding is that if =
the
> characterstic impedance is changing significantly with frequency you are =
probablly
> above the cable's usable frequency range.

The other option is that the source and load are imperfectly matched, and l=
arge changes in effective  impedance at the end of the cable can be seen du=
e to the length being close to an odd integer multiple of a 1/4 wavelength =
at the frequency of measurement.
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