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Re: [TowerTalk] Ledge Lightning Ground Advice

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Ledge Lightning Ground Advice
From: "Roger (K8RI) on TT" <K8RI-on-TowerTalk@tm.net>
Date: Mon, 2 May 2016 15:22:47 -0400
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
As ground rod diameter makes little difference, I'd think a "interconnected network" of bare #2 copper wire would be much more effective than ground rods laid horizontal. IIRC the NEC recommends larger, but most of us have budgetary limitations. With bed rock only 2 or 3 feet down, the area covered by the grounding network becomes important. I have over 600 feet of bare #2 and 32 or 33 ground rods Cadwelded to the copper in my network. I hasten to add that Copper was purchased BEFORE the price of copper went up.

Here the soil is much better for creating effective ground systems.
If the soil AND rock are dry (I believe Oregon receives plentiful rain) the area covered by the grounding system can be "doped" and kept moist. The larger the area the better as is the amount of heavy wire.. Normally a radial ground is run out from each tower leg up to 75 feet with ground rods spaced twice their length.. It's reported that little or nothing is gained going longer than 75 feet.

73

Roger (K8RI)

On 5/1/2016 Sunday 8:24 PM, Kevin Stover wrote:
I have seen 10' x 5/8 rods laid flat in trenches 2 or 3 feet deep in situations with shallow rock ledges.

On 5/1/2016 3:38 PM, Roger (K8RI) on TT wrote:
Some thoughts:

Much depends on the rock composition and moisture content.
If the soil and rock are primarily dry, the short ground rods probably offer little protection. You could pour a large pad of concrete with embedded rerod as a UFER ground, but that would likely be expensive. The other option would be a network of buried copper wire which can also get kinda pricey.

I'm no expert on UFER grounds so I don''t know how much would be adequate.

http://www.psihq.com/iread/ufergrnd.htm
https://en.wikipedia.org/wiki/Ufer_ground

(Quote below from Wikipedia)
"Ufer grounds, when present, are preferred over the use of grounding rods. In some areas (like Des Moines, Iowa) Ufer grounds are required for all residential and commercial buildings.[5] The conductivity of the soil usually determines if Ufer grounds are required in any particular area." (

There are usually a number on here well versed in the use of UFER grounds. Your base for the 25G is far more than what is needed. With a guyed tower, the base only serves one function: To keep the base of the tower in place, by preventing the base from skidding sideways and preventing it from sinking into the soil. If the concrete block is large enough it can also serve as a UFER ground. With solid rock only 2 to 3 feet down the sinking is solved. 3 holes for solid rod (epoxied in) to match the tower legs would solve the skidding problem

In days gone by, I have had a number of 40 and 50' towers with plain old "dirt bases" and large antennas. 5L on 20, 6L on 15 with 42' booms and 7L on 10. (39' boom) Admittedly 40' for 5L on 20 is low, but it was all I had.

ROHN even sold a "dirt base" for years, but I've not seen it offered in many years. Whether from abuse, or the lawyers (I suspect the latter) ROHN considered a dirt base adequate for many 25G installations. I'd not hesitate to use one for a 25G up to 60, or possibly 70 feet depending on the load weight and soil composition. I'd not go beyond that due to the weight as well as guy weight and tension adding to the weight pushing down. With taller towers the upper guys are longer and at a steeper angle which puts more of the tension onto the tower as a vertical load.

73

Roger (K8RI)


On 5/1/2016 Sunday 1:11 PM, David Merchant wrote:
Guys,


I could use some advice. Late last fall, I erected 40' of Rohn 25G with a Tilt-over base. It sits on top of a ledge that only has about 2-3 feet of rocky topsoil. When I dug the hole for the foundation, I excavated all the
loose fill and I core-drilled the rock below about 36".  I then epoxied
4-pieces of rebar in the holes, and poured about 3-1/2 yards of concrete with anchor bolts for the base plate. Unfortunately, I didn't think to bond
any of the rebar or anchor bolts.


Right now, the only grounding the tower has is a connection to a 2-ga ground wire that runs 120' underground with the conduit that feeds the power. It's
in-turn mechanically connected to a 2-4' ground rod (the max I could
install) every 15'. The 2-ga wire is also connected to the Single-Point
Ground  where the coax enters the home.


Since lightning season is about to get started, I'm concerned that this is
insufficient grounding.


I was considering bonding some 2-4" copper strap to each of the tower legs and running it along the ground for 50' or so. Also, I do have a SteppIR BigIR vertical about 20-feet away that has an extensive ground plane, albeit with insulated 12-14-ga wires. This could be bonded as well, but currently
isn't.


What would you guys recommend I do to augment the existing ground?


73,


Dave

K1DLM



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73

Roger (K8RI)


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