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.
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.
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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