[TowerTalk] There's 'ground', and then there's 'ground'

[Frank Donovan]

Keith,

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.

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.

73!
Frank
W3LPL
donovanf at erols.com


----- Original Message -----
From: Keith Dutson <kjdutson at earthlink.net>
To: <towertalk at contesting.com>
Sent: Sunday, January 16, 2005 3:59 PM
Subject: RE: [TowerTalk] There's 'ground', and then there's 'ground'


> >Its been well proven that braided strap has far too much inductance to be
> of any utility above 2 MHz.
>
> OK.  So where is the proof?
>
> I am thinking any writing you can find is based on the use of braid that
of
> poor manufacture or has corroded, or both.  Poor or corroded braid can act
> like many very small wires to create substantial impedance.  However, well
> constructed braid acts more like a solid bar of metal, but with the
> flexibility required for many installations.
>
> Keith NM5G
>
> -----Original Message-----
> From: towertalk-bounces at contesting.com
> [mailto:towertalk-bounces at contesting.com] On Behalf Of Frank Donovan
> Sent: Sunday, January 16, 2005 10:35 AM
> To: towertalk at contesting.com
> Subject: Re: [TowerTalk] There's 'ground', and then there's 'ground'
>
> Here's a clarification, pointed out by an attentive towertalkian:
>
> My previous e-mail was not intended to refer to cables that use a braided
> shield under a jacket.  A tight jacket compresses the braid and it's a
very
> effective RF conductor, provided that the braid is not corroded.
>
> Rather, my comment was intended to apply to to the inappropriate use of
> braided strap for RF grounding or lightning protection.  Its been well
> proven that braided strap has far too much inductance to be of any utility
> above 2 MHz.
>
> So...   Lets try again:
>
> Discard all forms of "braided" strap.
> Braid is something a girl does to her hair.
> Braided strap is a poor RF conductor and it's a terrible conductor of
> lightning energy.
>
> Use braided strap only for DC and audio applications, its just an inductor
> for RF or lightning protection applications.
>
> Here's an excellent reference from the many industry sources on this
topic:
> http://members.cox.net/pc-usa/station/ground5.htm
>
> 73!
> Frank
> W3LPL
> donovanf at erols.com
>
>
>
> _______________________________________________
>
> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
> Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with
any
> questions and ask for Sherman, W2FLA.
>
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> _______________________________________________
>
> See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
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