An example in one of Polyphasers books shows, with 10 foot spacing of rods,
2 rods gives about 50% of one.
3 rods drops to 40%.
4 rods to 35%.
5 rods to 30%.
6 rods to 27%.
7 rods to 25%.
8 rods to 23%.
With closer spacing there is not as much gain in resistance reduction.
5 ft spacing for 4 rods reduces it to only about 42%.
Keep in mind this is for a particular soil conductivity.
This is from their "grounds for lightning and EMP protection book".
73
Gary K4FMX
Keith Dutson wrote:
I read Polyphaser's technical note before installing my system. There is a
lot of discussion about soil conductivity and how adding more rods will
improve conductivity. Do you have an idea of HOW MUCH impedance would be
reduced?
Keith NM5G
-----Original Message-----
From: Gary Schafer [mailto:garyschafer@comcast.net]
Sent: Monday, January 17, 2005 10:39 AM
To: keith@dutson.net
Cc: towertalk@contesting.com
Subject: Re: [TowerTalk] There's 'ground', and then there's 'ground'
Hi Keith,
2 ground rods is not even a good start for a ground to be blunt.
Running some radials out from your connection point and adding more rods
will help lower the impedance of the ground system.
A look on Polyphasers site will provide some good insight as to layout and
how many you should use for your situation.
Unless you are only concerned with a safety ground I would add more ground
system.
73
Gary K4FMX
Keith Dutson wrote:
There are already two 8 foot rods just outside the window. Why would
I need more? And if the argument is for better grounding, please
indicate your thoughts of the percentage of reduction in impedance I
would get in Texas Gulf Coast sandy loam (5 feet over iron ore clay).
Thanks.
Keith NM5G
-----Original Message-----
From: Gary Schafer [mailto:garyschafer@comcast.net]
Sent: Monday, January 17, 2005 9:49 AM
To: keith@dutson.net
Cc: towertalk@contesting.com
Subject: Re: [TowerTalk] There's 'ground', and then there's 'ground'
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
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with any questions and ask for Sherman, W2FLA.
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