Re: RF Ground Connections - Wire Type

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Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>


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From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Tuesday, July 16, 2002 5:16 PM
Subject: RE: RF Ground Connections - Wire Type


> Original poster: "Mccauley, Daniel H by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <daniel.h.mccauley-at-lmco-dot-com>
>
>
> I'm going to have to disagree with your assessment.  For high
> voltage transients, stranded or braided wire outperforms solid wire in all
> instances.  This is the primary reason you use braided wire for lightning
> protection in house security systems etc...  Another example is the use of
> stranded wire vs. solid in audio applications.

You have to be careful about the basis for comparison.. Same AWG?  Clearly,
a 5 inch diameter solid bar of copper will work better than a stranded #28
wire from an electrical resistance/impedance standpoint, but is somewhat
more costly and hard to work with.  Or, the other way around, 4/0 welding
cable is probably better than 30 AWG wirewrap wire in some instances.

Stranded wire is used in security and protection systems because it is easy
to bend, and you are less likely to get an "intermittent open" from one
broken strand.

No question that ordinary uninsulated braid is higher RF resistance than the
same cross sectional area of a solid bar (or a tube), because of the "wire
to wire" surface resistance, over and above the skin effect.  There's quite
a bit of literature on this.

Litz wire, where the strands are individually insulated, is another matter
entirely.

Over all, though, I suspect that the kind of ground wire you use isn't
nearly as important as its length.

>
> > My recent measurements of conductor AC resistance confirm that stranded
> > wire is a POOR choice.  While one should strive for a high surface area,
> > this only applies to solid conductors.  RF currents want to travel only
on
> > the outermost surface - they will not travel on strands inside the
bundle.
> > When a strand goes from the surface of the bundle to inside the bundle,

> > the
> > current will try to hop to an outer strand, through whatever surface
> > oxides
> > might be in its path.  I did not compare finely stranded conductors with
> > coarsely stranded conductors, but I would speculate that a coarsely
> > stranded conductor might have a lower AC resistance, as it's a closer
> > approximation to a solid conductor.
> >