On 6/13/12 2:46 AM, John Langdon wrote:
> Start with your soil: low or high conductivity, wet or dry, as the basis for
> decisions.
>
> You are ahead to recognize there are two grounds: one for lightening
> protection, the other for radio frequencies.
>
> If you have very low conductivity soil, like limestone, then for lightening
> protection, I think about 'spreading the charge out' and thus look for
> maximum surface area and spreading out the rods. I use copper strap (much
> better than large wire) with a 10' deep rod about every 10 feet, using
> Cadweld to bond them together, and the more the better. In my soil, you
> cannot drive a rod, you must drill a hole and back fill it with Bentonite
> clay. The more the better, but absolute minimum is three straps with at
> least two rods each. Remember this is for very low conductivity soil or
> rock.
For lightning (but not RF) strap does not provide a significantly better
ground than wire. Lightning grounding is all about inductance, not AC
resistance, and the inductance isn't greatly affected by the shape of
the conductor (two wires parallel and adjacent to each other have about
the same inductance as a single wire).
If the soil is low conductivity, use what Herb Ufer developed for
specifically this situation (grounding explosive igloos in the desert)..
20 feet of wire in concrete. The concrete has far more contact area
than any set of wires or rods, and it is higher conductivity than the
surrounding soil (partly because it's hygroscopic, partly because of the
chemical composition).
Big contact area does three things (compared to rods):
reduces the resistance
reduces the impedance (particularly in dry soil, it's more like a
capacitor, so concrete is a bigger plate than a rod)
reduces the current density across the interface
I don't have the calculations here at hand, but compare the surface
contact area of a 20 foot long concrete bar that's 1 foot deep and 1
foot wide to any collection of rods and wires.
The bentonite and rod in a hole scheme basically is an approximation of
this, but smaller (drive 8 feet of rod in a 1 foot diameter hole with
bentonite, and it's electrically much like a 1 foot diameter concrete
rod with an 8 foot wire in it). If you're in a situation where you
can't trench and lay concrete, but you can drill a hole, then the
rod/bentonite scheme works. (bentonite, like concrete, is also
hygroscopic and high conductivity, because of the very fine
particles..wet clay soil is much lower resistivity than wet sand)
Rod+bentonite is a handy engineering solution when you are doing things
like planting telephone poles: you already have the post hole digger
handy, you have a bunch of sacks of bentonite on the truck, etc. It's
also handy if you're putting a structure on a rocky outcrop. You
already have the drill there for the chemical anchors, so drilling some
more holes for grounding is a logical approach.
BTW, if you have a tower base that's 4x4x4 feet concrete cube or
something, you already have plenty of lightning ground. That's 80
square feet of contact area already. Put that 20 foot wire in the
concrete (or bond to the rebar cage, or both).
>
> If you have high conductivity soil, then make sure there are few if any
> dissimilar metal junctions, as wet clay will dissolved them in a few months.
> For radio grounds, a large number of smaller wires will work.
This is the "vertical antenna" RF ground aspect.. strap, lots of wires,
low AC resistance, low RF power loss. For lightning, it doesn't make as
much difference.
>
> Tie all the feed line outer conductors directly to ground near the base of
> the tower, and bond that point to the tower with a stainless steel surface
> between the galvanized tower and the copper, seal that connection from the
> weather as well as possible, and then use Polyphaser protectors there before
> burying the cable for the run to the shack. I am a belt and suspenders guy,
> so I use a perimeter ground at the shack, and another Polyphaser at the
> entrance panel.
>
> 73 John N5CQ
>
>
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of Rick Kiessig
> Sent: Tuesday, June 12, 2012 7:18 PM
> To: towertalk@contesting.com
> Subject: [TowerTalk] Tower grounds
>
> Is there a good rule-of-thumb for how many ground rods to use for a tower,
> and how deep they should be? Does the diameter of the rods make any
> difference? Is it worth keeping grounds for lightning protection (tower
> ground) separate from RF grounds (tie the coax shield to ground)?
>
>
>
> 73, Rick ZL2HAM
>
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