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

Mon Jan 17 09:37:56 EST 2005

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 at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Frank Donovan
Sent: Monday, January 17, 2005 6:23 AM
To: towertalk at 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 at erols.com

----- Original Message -----
From: Keith Dutson <kjdutson at earthlink.net>
To: <towertalk at 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 at contesting.com 
> [mailto:towertalk-bounces at contesting.com] On Behalf Of Tom Rauch
> Sent: Sunday, January 16, 2005 8:26 PM
> To: keith at dutson.net; 'Frank Donovan'; towertalk at 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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> _______________________________________________
>
> 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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