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Braid RF resistance... Re: [TowerTalk] There's 'ground',and then there's

To: "Frank Donovan" <>, <>
Subject: Braid RF resistance... Re: [TowerTalk] There's 'ground',and then there's 'ground'
From: "Jim Lux" <>
Date: Sun, 16 Jan 2005 14:07:59 -0800
List-post: <>
> Braid acts exactly as you suggest only when the many small
> diameter conductors are under sufficient pressure to cause the
> RF currents to flow on the surface of the braid.  This is the
> case with the shield of coaxial cables or other shielded cables
> enclosed in a jacket.  The pressure of the jacket causes the
> RF currents to "jump" from one braid wire to the next so that it
> stays on the surfaces of the coaxial cable shield.  When water
> enters a coaxial cable, the wire junctions in corroded shield
> become lossy and the cable loss increases significantly.
> Braid straps have no pressure to cause RF currents to reliable
> flow on the outer surface of the braid strap.  As a result, braid
> straps have significantly higher inductance than any of the more
> appropriate alternatives for RF conductors, such as solid copper
> strap, copper tubing, solid copper wire or stranded wire.
> Braid straps work well at low frequencies, but they behave as
> inductors at RF frequencies.

Maybe not "inductance" here?  RF resistance or impedance might be a better
way to approach it.  I think what's going on is a combination of skin
effect, etc.  A bunch of parallel (insulated) wires might have low
inductance but high RF resistance.

> I'll leave the proof to the student; however, you can find source
> material on this topic in RF engineering and EMC
> (electromagnetic compatibility) text books.
> You can also find information about good grounding and bonding
> practices on the web if you spend a few minutes spent Googling
> for: "grounding and bonding" EMC.

The curse of the web is that such a google will turn up a huge number of
links, some providing information of dubious accuracy, leaving an awful lot
of "exercise for the reader".

Very, very few sites actually provide any analysis to back up their
statements, so that a reader can evaluate what they're saying.  Most sites
of this type are merely rehashing "common wisdom" (which might be wrong or
obsolete) or simply present assertions as fact, without backup. Or, the site
is provided by someone with an obvious interest in the results (i.e. a
seller of equipment), so, while the data may be perfectly correct, it's not
provided by an impartial third party.

Finally, even if you drag out the IEEE specs on such things, they require a
fair amount of  analysis to apply them to a particular implementation or
situation. (this is, after all, what engineers get paid to do)

If there were a site that provided such data, with unambiguous analysis,
this online discussion probably would have ended a long time ago.

Jim, W6RMK


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