I'm installing a new tower and bought, as part of the package, the
grounding kit. This consists of four 10 ft copper clad ground rods,
plus all the accessories needed to connect them to the tower.

My problem is that after about 5 feet of good old red clay I hit solid
rock. I'm tempted to cut the 10 ft rods in half and simply have eight
5 ft rods. Since this is the only practical alternative that I can
see.

Will the eight 5 ft rods give me the same "ground" as the four 10 ft
rods? Or, do I need to add even more 5 ft rods to achieve the same
ground? ( I knew I should have bought that Megger at the hamfest. )

Opinions welcomed. Empirically derived evidence treasured! 8*)


Ken K4XL


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On Tue, 02 Oct 2007 11:58:46 -0400, "Kenneth Grimm, K4XL"
wrote:

Will the eight 5 ft rods give me the same "ground" as the four 10 ft
rods? Or, do I need to add even more 5 ft rods to achieve the same
ground?

Hi Ken,

Oddly enough, you could as easily lay the rods out in a shallow trench
(say, a foot deep) and get the same grounding, if not better. The
only rule of thumb that might be observed is to not have them in
proximity to one or the other. Proximity would mean about five feet
in 10 foot parallel trenches.

However, to satisfy convention: cut them in half (or research the
archives here for rotary hammers) and keep them isolated, one from the
other, by at least three feet.

73's
Richard Clark, KB7QHC

"Kenneth Grimm, K4XL" wrote in message
...
I'm installing a new tower and bought, as part of the package, the
grounding kit. This consists of four 10 ft copper clad ground rods,
plus all the accessories needed to connect them to the tower.

My problem is that after about 5 feet of good old red clay I hit solid
rock. I'm tempted to cut the 10 ft rods in half and simply have eight
5 ft rods. Since this is the only practical alternative that I can
see.

Will the eight 5 ft rods give me the same "ground" as the four 10 ft
rods? Or, do I need to add even more 5 ft rods to achieve the same
ground? ( I knew I should have bought that Megger at the hamfest. )

Opinions welcomed. Empirically derived evidence treasured! 8*)


Ken K4XL


*** BoatAnchor Manual Archive ***
On the web at http://bama.sbc.edu and http://bama.edebris.com
FTP site info: bama.sbc.edu login: anonymous p/w: youremailadr

Ken

This very subject was covered at some length not so long ago. The consensus
was that solid rock does not make a very good earth. Driving the rods in at
an angle to get the full 10 feet under the soil is probably going to be
adequate. If you are concerned that the earth is not good enough, add some
lengths of thick, bare copper wire buried in the soil and conected back to a
common earth point. The rods are mainly there to provide a dissipation path
for lightning if it should hit the tower.

Mike G0ULI

Kenneth Grimm, K4XL wrote:
Opinions welcomed. Empirically derived evidence treasured! 8*)

Think outside the box - drive them into the ground
at a 30 degree angle.
--
73, Cecil http://www.w5dxp.com

On Tue, 02 Oct 2007 10:21:42 -0700, Richard Clark
wrote:

Oddly enough, you could as easily lay the rods out in a shallow trench
(say, a foot deep) and get the same grounding, if not better.

followed by comments from Mike and Cecil suggesting driving the rods
at an angle.

Thanks to all of you for encouraging me to think outside the box! Now
all I have to do is find a sledge with a 30 degree adjustment on it.
Actually, most of the nails I've driven have gone in at about that
angle, so I shouldn't have any trouble at all. 8*)

Thanks again. 73,

Ken K4XL


*** BoatAnchor Manual Archive ***
On the web at http://bama.sbc.edu and http://bama.edebris.com
FTP site info: bama.sbc.edu login: anonymous p/w: youremailadr

Jimmie D wrote:
"Richard Clark" wrote in message
...
On Tue, 02 Oct 2007 11:58:46 -0400, "Kenneth Grimm, K4XL"
wrote:

Will the eight 5 ft rods give me the same "ground" as the four 10 ft
rods? Or, do I need to add even more 5 ft rods to achieve the same
ground?
Hi Ken,

Oddly enough, you could as easily lay the rods out in a shallow trench
(say, a foot deep) and get the same grounding, if not better. The
only rule of thumb that might be observed is to not have them in
proximity to one or the other. Proximity would mean about five feet
in 10 foot parallel trenches.

However, to satisfy convention: cut them in half (or research the
archives here for rotary hammers) and keep them isolated, one from the
other, by at least three feet.

73's
Richard Clark, KB7QHC

I think the rule of thunb is to separate ground rounds by an amount equal to
their length.

Which gets pretty tought as you bring them to the antenna..... ;^)

-73 de Mike KB3EIA -

Kenneth Grimm, K4XL wrote:
"Will the eight 5 ft rods give me the same "ground" as the four 10 ft
rods?"

Maybe better.

Skin effect depends upon conductivity. On copper it is measured in
thousandths of an inch but in the earth`s surface, penetration may be 10
or 20 feet.

Skin thickness is also inversely proportional to the square root of the
frequency but does not change much sbove about 6 MHz due to electrical
capacitance in the soil which makes it appear more of a lossy
capacitance (dielectric) than a poor conductor.

Soil with a conductance of 6 mmho/meter has a dielectric thickness of
about 8 feet at 6 MHz. A ground rod which would reach the depth of
energy penetration is recommended. E.A. Laport says at the end of page
49 of "Radio Antenna Engineering": "Ideally, the ground rods should
reach down to the depth corresponding to the skin thickness for the soil
conductivity and the operating frequency."

However, using eight 5 ft rods instead of four 10 ft rods would likely
improve a grounding system especially if the rods are placed some
distance away from their common connection point, say 10 feet or more.
The interconnection wiring acts somewhat as a radial system and will
have substantial capacitive coupling to the earth at freuencies where
the earth`s dielectric properties prevail.

Best regards, Richard Harrison, KB5WZI

"Kenneth Grimm, K4XL" wrote in
:

I'm installing a new tower and bought, as part of the package, the
grounding kit. This consists of four 10 ft copper clad ground rods,
plus all the accessories needed to connect them to the tower.

My problem is that after about 5 feet of good old red clay I hit solid
rock. I'm tempted to cut the 10 ft rods in half and simply have eight
5 ft rods. Since this is the only practical alternative that I can
see.

Will the eight 5 ft rods give me the same "ground" as the four 10 ft
rods? Or, do I need to add even more 5 ft rods to achieve the same
ground? ( I knew I should have bought that Megger at the hamfest. )

Ken,

Some thoughts...

The main reason for using driven electrodes at the tower base is for
lightning protection. I assume that is your purpose.

To predict the effect of driving a number of electrodes, and the depth of
driving, you have to consider the effect of the soil. It has high
resistivity, and resistivity often changes with depth. Clay soils are
likely to be drier at the surface, and higher in resistivity.

So, longer electrodes are often more effective because they reach down to
wetter layers, and it is not unusual that an 8' electrode has a quarter
the resistance of a 4' electrode for that reason.

Hitting rock isn't necessarily all bad, it sometimes happens that the
rock causes wetter clay above it, and shorter electrodes may be quite
effective.

You can assess the likely outcome by driving two separated electrodes and
measuring the resistance. You can estimate the benefit of driving further
adequately spaced electrodes (assuming uniform soil).

Dont overlook using the reinforced concrete base of the antenna as part
of your ground system.

A megger! No, if your earth system is any good, a megger isn't going to
be useful. There are a number of techniques for measuring the effective
low frequency resistance of an earth electrode or earth system. Google
for things like the "three wire fall of potential method".

If you can't get sufficiently good earth from driving the electrodes as
is, choices include drilling (more effective in soft porous wet rock) or
strip electrodes laid in trenches.

As for the 30deg sledge hammer, I put some notes and a pic together on
tools for driving earth electrodes, see
http://www.vk1od.net/post/driver.htm . The smallest tool was fabricated
after recent discussion on using an SDS Rotary Hammer to drive earth
electrodes.

Owen

On Tue, 02 Oct 2007 14:54:03 -0400, Jimmie D wrote:

"Richard Clark" wrote in message
...
On Tue, 02 Oct 2007 11:58:46 -0400, "Kenneth Grimm, K4XL"
wrote:

Will the eight 5 ft rods give me the same "ground" as the four 10 ft
rods? Or, do I need to add even more 5 ft rods to achieve the same
ground?

Hi Ken,

Oddly enough, you could as easily lay the rods out in a shallow trench
(say, a foot deep) and get the same grounding, if not better. The only
rule of thumb that might be observed is to not have them in proximity
to one or the other. Proximity would mean about five feet in 10 foot
parallel trenches.

However, to satisfy convention: cut them in half (or research the
archives here for rotary hammers) and keep them isolated, one from the
other, by at least three feet.

73's
Richard Clark, KB7QHC

I think the rule of thunb is to separate ground rounds by an amount
equal to their length.

Going by what I read on the Polyphaser site some time back and what BICSI
recommends for telecom grounding, a minimum distance is eight feet
between ground rods. Closer than that and the rods won't be able to
dissipate the energy effectively.

Also, more shorter rods covering a larger surface area of earth is
preferable to fewer deeper rods. Minimum length is considered to be 5
feet.

Finally, be sure to tie the tower, shack, and AC entrance grounds
together for a "single point" ground to avoid ground potential
differences between interconnected equipment.

73, de Nate

--

"The optimist proclaims that we live in the best of all possible worlds,
the pessimist fears this is true."

On Tue, 2 Oct 2007 14:54:03 -0400, "Jimmie D"
wrote:

I think the rule of thunb is to separate ground rounds by an amount equal to
their length.

Hi Jimmie,

Quite so.

73's
Richard Clark, KB7QHC