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

To: keith@dutson.net
Subject: Re: [TowerTalk] There's 'ground', and then there's 'ground'
From: Gary Schafer <garyschafer@comcast.net>
Reply-to: garyschafer@comcast.net
Date: Mon, 17 Jan 2005 10:49:05 -0500
List-post: <mailto:towertalk@contesting.com>
If you want to spend additional money to add to what you have, I would spend it on a few more ground rods instead.

73
Gary  K4FMX

Keith Dutson wrote:
AES is the vendor for my Harger bar.  The bar was shipped with a few short
braids and stainless hardware for mounting.  I almost included that roll of
3 inch copper in the order but figured the 1.5 inch braid in my spare parts
bin (shown in my earlier post) would do just as well.  Think I will place
that copper strap order now.  Shame that I had to buy those huge copper lugs
and use a propane torch and about half a roll of solder to get a good bond.

BTW, the Harger bar is 1/8 thick and about a yard long, with drilled/tapped
holes for light straps and larger holes for bolting heavy straps.  It is
mounted with bolts through Teflon standoffs.  These bolts are not supplied,
but several stainless nuts, bolts and lock washers are included in the kit.
Price from AES is about $70 with shipping.

Keith NM5G

-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Frank Donovan
Sent: Monday, January 17, 2005 6:23 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] There's 'ground', and then there's 'ground'

Keith,

You should use braided strap only when RF grounding or lightning protection
is not required.  Otherwise, use copper strap, copper tubing or stranded or
solid copper wire.
If you must use braid, use the 1 inch wide braid, and limit its length to
not more than one foot.

In the Amateur Electronic Supply catalog, 1.5 inch wide copper strap is
priced ($99.99/100 ft) about the same  as
1 inch wide braid ($92.99).   The 3 inch copper strap is
even better and is listed in the AES catalog at $139.99 per 100 feet.

73!
Frank
W3LPL
donovanf@erols.com

----- Original Message -----
From: Keith Dutson <kjdutson@earthlink.net>
To: <towertalk@contesting.com>
Sent: Monday, January 17, 2005 12:13 AM
Subject: RE: [TowerTalk] There's 'ground', and then there's 'ground'



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.

Keith NM5G

-----Original Message-----
From: towertalk-bounces@contesting.com [mailto:towertalk-bounces@contesting.com] On Behalf Of Tom Rauch
Sent: Sunday, January 16, 2005 8:26 PM
To: keith@dutson.net; 'Frank Donovan'; towertalk@contesting.com
Subject: Re: [TowerTalk] There's 'ground', and then there's 'ground'



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 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.

73 Tom


_______________________________________________


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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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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_______________________________________________

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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