[TowerTalk] Ledge Lightning Ground Advice

[Kevin Stover]

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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>
>


-- 
R. Kevin Stover
AC0H
ARRL
FISTS #11993
SKCC #215
NAQCC #3441


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