A few months ago on this reflector, Dick, W6OLD, published some comments
regarding the grounding approach he took for his tower and antenna system.
Recently, I asked him some questions about his installation. His comments
were certainly of interest to me, and I thought they might also be beneficial
to the group:
73 - Phil, N6ZZ
------------------------------------------------------------------------------
-----------------------------------------
Subj: Re: Fwd: Ufer grounding system
To: <towertalk@contesting.com>
Date: 98-02-24 23:26:27 EST
From: dick@libelle.com (Dick Flanagan)
To: N6ZZ@aol.com
>Back in September, you described some of the details of your Ufer ground
>system on the Towertalk reflector. I'm about to move from my Dallas QTH to
>the mountains of Southern New Mexico, and I will be putting up a tower! A
>couple of questions regarding your foundation system:
I'll do the best I can.
>1. You mentioned having 7 50-foot radial grounds, spaced 30 degrees apart
The reason for the 180 arc is simply because the geometry of our lot
doesn't allow me to go in the full 360 circle that is common for this
application. I used the 180 arc to direct the absorbed lightning energy
away from the house.
> A. What kind of wire are you using for the radials?
I used 1.5" copper strap for maximum surface area/contact with the
surrounding soil.
> B. How are the pigtails bonded to the radials?
I used the Polyphaser strap clamps (as you found in the catalog)
> C. How far below surface are the radials buried?
Approximately 18 inches.
> D. I think you've mentioned in other posts that your have hard,
>rocky soil.
> If that is the case, how did you bury the radials?
With difficulty. :)
Originally I had hoped to dig the radial trenches and sink eight-foot
ground rods every 12 feet or so and then connect those rods with the copper
strap. The soil here isn't so much rocky as it is sandy over a very thick
layer of hardpan about two feet below the surface. As it turned out, the
hardpan was simply too hard to get the rods through. :(
After talking with two Polyphaser engineers, I decided to omit the rods and
simply lay the strap along the bottom of the trenches. I flooded the
trenches to ensure good soil/mud contact with the strap and then just
filled them in. In retrospect, if I had known I was going to just bury the
strap, I would have rented a "ditch witch" for a six-inch wide trench,
instead of a back hoe.
>2. You mentioned that you have 8 pigtails of 2/0 wire attached to the rebar.
> A. Are these just clamped to the rebar, like with Polyphaser's
>bronze transition clamps?
No, I used 3/8-inch wire clamps and liberal amounts of copper antioxidation
agent. I used a drill-driven wire brush to clean the rebar down to bare
metal, laid about six inches of 2/0 pigtail parallel against the rebar and
bonded the two together with two of the wire clamps. I used the copper
antioxidation compound around the rebar and pigtail to minimize dissimilar
metal problems. I also used a lot of force on the wire clamps to ensure a
strong mechanical bond.
The copper antioxidation compound is similar to the old reliable OxGuard
you get at the hardware store, but it uses suspended copper, instead of
suspended aluminum. It comes with Polyphaser's Copper Cleaning Kit.
>3. You seem to have paid a great deal of attention to the foundation part of
>your tower installation. I noticed that Polyphaser mentions that their 1.5"
>strap has lower inductance than 2/0 cable. That being the case,
> A. Why did you opt for the 2/0 cable rather than the strap?
Physical strength and durability. The pigtails would have to withstand the
force of the concrete being poured in the hole and would be exposed to
human and natural elements where it came out of the concrete and was bonded
to the tower, itself. The strap is nice, but it can be fairly easily
twisted and torn by rough treatment. As soon as the pigtails cleared the
concrete at the trenches, I bonded them to the strap which was then
protected by being buried.
If you imagine the rebar cage, picture the top horizontal ring of rebar
about six inches below the surface of the concrete. Now picture the second
horizontal ring about a foot below the first. It was this second ring that
all the pigtails were bonded to so when they exited straight out of the
concrete, they exited about 18" below the ground, into the base of each
trench. I also attached two more 2/0 pigtails to this "ground ring."
These two came out the top of the concrete pad: one to bond to the tower
base and one to bond to a ground plate mounted within a weather proof box.
This ground plate was used for any secondary ground/protection devices that
might be wanted later. For example, I have a spare piece of coax
terminated in a Polyphaser coaxial protector mounted on this ground plate.
> B. If you had gone with the strap rather than the cable, how would
>you have fastened it to the rebar and the radials? (Maybe that's why you
>went with cable rather than the strap in the first place!)
Again, I would have used the Polyphaser clamps.
As for the rest of the external grounding, in addition to the seven buried
strap radials, I have an eighth strap running about 85 feet back to the
shack. It is in a trench that also contains the Sched 40 PVC containing
the coax and other wires. Where it reaches the shack, the strap connects
to a length of 3" copper strap that runs about 30 feet to the service
entrance ground. That 3" strap also runs up into a 24 X 24" weatherproof
box on the outside of the shack wall where it connects to a copper plate
lining the box, as well as running through the wall to provide the central
shack ground.
The coax is lightning protected at the top of the tower, at the bottom of
the tower and at the shack entrance panel. The rotor and other control
wires are protected at the top of the tower and at the shack entrance
panel. The 110 VAC winch wires are protected at the shack entrance panel.
All in all, we've got about $9,000 invested in the tower installation:
$7,000 for the tower, $1,000 in the footing (16 yards of concrete) and
excavation, and $1,000 in the lightning protection devices. When you
figure the value of all the equipment being protected, not to mention the
personal protection, it is money well spent.
As you mentioned, I've given this a fair amount of thought and would be
happy to share whatever I've learned with you. If you have any other
questions or concerns, I would be happy to discuss them.
73, Dick
--
Dick Flanagan W6OLD CFII Minden, Nevada DM09db (South of Reno)
Visit http://www.qsl.net/w7di/
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search
|