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Re: [TowerTalk] tower & antenna project questions

To: "Tower and HF antenna construction topics." <>
Subject: Re: [TowerTalk] tower & antenna project questions
From: jimlux <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Thu, 11 Mar 2010 22:43:40 -0800
List-post: <">>
>> I also need to install an electrical outlet near the base for the tower 
>> motor. So I will be running a 3/4" conduit as well, from the main panel 
>> out to the tower base. Any objection to dropping the electrical conduit in 
>> the same trench as the coax conduit?
> COMMENT:  Good practice would have at least a foot separating the coax and 
> power cables.  With the cables in conduits, I'm a bit murky about what the 
> actual separation should be.  But to be safe, put the coax conduit in the 
> bottom of the trench, then cover with a foot of soil, then lay the power 
> conduit and fill in the remainder.

I'd put the conduit with the AC line in the bottom, and the coax above 
it.  It depends on your local area how deep they want the conduit with 
the AC in it. 18" is typical.

Remember that, in theory, the conduit with the AC above ground level has 
to be rigid metallic, so you have to transition at some point.  A PVC 
sweep to a female threaded and then finishing with a section of rigid 
conduit is one way.  Your local rules may be different.

There's no minimum separation between conduits that I can recall off the 
top of my head. You just can't run AC power and something else in the 
SAME conduit or bring them into the SAME box. Unless there is a 
mechanical divider in the box, like you see for sprinkler timers and 
landscape lighting.

>> Finally, as far as grounding goes - is there any advantage to installing 
>> three 8ft ground rods at the bottom of the tower foundation hole BEFORE I 
>> pour the concrete, and run the ground wires up thru the concrete base, to 
>> be attached to the tower once it is erect? Or just stick to three ground 
>> rods into the dirt around the base of the tower foundation after it is 
>> done?
> COMMENT:   Placing three ground rods that close together in the bottom of 
> the excavation is a waste of copper. Lightning energy will have to soak into 
> the same small volume of earth underneath the foundation.  Separating the 
> rods will dump the lighning energy into DIFFERENT volumes of earth, or at 
> least MORE earth, so the energy density is less.  You will get more 
> effective grounding by separating the rods in earth OUTSIDE of the 
> foundation.

Put your copper wire into the foundation and end it there.  It should 
only penetrate the air/concrete boundary above grade.  If you want to 
put ground rods in, too, put them into the soil outside the concrete.

But concrete is a pretty good conductor, and has a LOT more surface area 
than a 1/2" diameter rod.  When you're talking dissipating lightning 
energy, you want to spread it out.


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