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

To: <>, "'Frank Donovan'" <>,<>
Subject: Re: [TowerTalk] There's 'ground', and then there's 'ground'
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Sun, 16 Jan 2005 21:25:41 -0500
List-post: <>
> However, it is empirical evidence that I am seeking.  My
station ground
> system design is based on what I have found in ARRL and
other publications
> such as the technical documents provided by Polyphaser and
Harger.  I found
> nothing published that spoke of the adverse effects caused
by using braided
> straps.  Your posts are the only source.

Sometimes we can't easily find things that are commonly
known or understood in engineering circles. Most engineers
and many hobbyists understand skin effect, and it only takes
a moment to reason through this.

The problem with any conductor at high frequency is skin
effect "pushes" current to the outside. When the conductor
is woven, current either has to flow from inward moving
strand to a surface stand through pressure contact or suffer
a path of greatly increased impedance if it follows the
original strand inside the braid.

The typical clean copper braid with a basically parallel lay
and minor weave has perhaps four times the resistance per
unit length of a similar width smooth surfaced conductor. Of
course it varies with the braid construction and contact
resistance between strands, but that is generally for better
braid that is clean with good pressure contact between

In HF power amplifiers, I have found a good general rule of
thumb is this:
At 30 MHz is the clean braid from RG-8 cable has about the
same current carrying capacity as #14 or 16 tinned solid
buss wire.

It's quite common to have braided leads of rather large size
overheat and fail even at just several amperes at radio

Many construction standards prohibit braided straps in RF or
lightning paths unless the connection absolutely must have
braiding in order to withstand flexing, and then the braid
is often substantially oversized to make up for its reduced
current capacity. It's my understanding NASA restricts use
of braiding, and I can cite many cases where braiding will
either cause excessive loss or actually fail in high current
RF systems.

By the way, that permanent loss increase you see in coaxial
cables that have been wet, even after they dry back out? It
primarily comes from loss of strand contact in the weave
caused by the tarnishing of conductors.

If the Handbook tells readers braiding (especially braiding
that might be exposed to moisture) is a good idea in high
frequency or lightning grounds....they are giving bad

73 Tom


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