OT: Re: [lug] cheap 802.11b for linux...

[J. Wayde Allen]

On Fri, 22 Mar 2002, D. Stimits wrote:

> > Shielding shouldn't directly affect loss.  At least, I can't see why ...?
> 
> Think of the skin effect at higher frequencies. Direct current flows
> through the center of a conductor, but as frequency goes up, electrons
> go to the outer edge of the conductor.

The electrons will go to the inside surface of the outer conductor, but
for this reason only this surface will see the field.  So ... a shield
foil or braid outside of this shouldn't really see much if any energy.  
At least not unless there is a hole in it.

> I suspect that as things go higher and higher in frequency, the
> physical shape becomes more important to the point that a perfect
> polished pipe might actually be more important than the braided
> surface, just because of the radiation patterns.

Yes, the physical shape becomes critical.  In fact, this forms the
foundation for the primary impedance standards here at NIST.  These are
precision airlines that have been carefully measured in our dimensional
metrology lab.  The characteristic impedance of the line is computed from
these dimensions.

Anyway, my guess is that it isn't so much the idea of having two so-called
shields as the quality of the braid.  I think this is along the lines of
what you are alluding to.  The better cables aren't braided but rather
continuously clad.  This starts getting you into the hardline talked about
earlier.

Then of course there is the idea of actual shielding.  This usually is a
secondary conductive layer that surrounds the core of the cable and acts
as an grounding plane to prevent external electric fields from coupling
into the cable.  That is a different topic and not related specifically to
carrying the signal in the cable.

Yes, it is probably time for me to start reigning this in - grin.

- Wayde
  (wallen at lug.boulder.co.us)