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

[D. Stimits]

"J. Wayde Allen" wrote:
> 
> 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.

By definition a braid is a rating on hole density :P

More and better braid means a flexible cable with a surface that
"appears" more polished to a given wavelength. Braided wire is just
sandpaper to RF.

> 
> > 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.

Me too :P It's a fun topic for me, you'd be surprised at what
transmission line theory applies to.

D. Stimits, stimits at idcomm.com

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