Now this is the most plausible explanation seen so far. Basically it seems
that there is no valid reason to have measured evidence if one follows sound
engineering practice. Makes perfect sense to me. Think I will replace the
braid with copper strap.
Tom, thanks for posting. And thanks to Frank for bringing up this issue.
As a side note, I went back to the Harger site which lists tinned copper
braid as one of their grounding strap options. All of their braid is flat
and tightly woven (not the flattened coax shield found at most vendors),
very heavy gauge, and is bonded to special sandwich type lugs to keep them
absolutely flat at each end. I am thinking this is a reasonable alternative
to wide strap when one must have the flexibility.
[mailto:email@example.com] On Behalf Of Tom Rauch
Sent: Sunday, January 16, 2005 8:26 PM
To: firstname.lastname@example.org; 'Frank Donovan'; email@example.com
Subject: Re: [TowerTalk] There's 'ground', and then there's 'ground'
> However, it is empirical evidence that I am seeking. My
> system design is based on what I have found in ARRL and
> 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 strands.
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 frequencies.
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 advice.
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.
TowerTalk mailing list
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.
TowerTalk mailing list