[TowerTalk] Ledge Lightning Ground Advice

[Roger (K8RI) on TT]

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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>
>
>
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-- 

73

Roger (K8RI)


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