The spacing between the towers is an important parameter and I could not
tell from your post how far apart they will be. I use a rule of thumb that
if the towers are more than 80' apart, they are essentially two separate
entities as far as lightning strikes are concerned, and they need two
separate ground systems which are not tied together. I also think each
tower should be 80' or more away from the point where the feed lines enter
the shack, and at that point you should have another ground system, which is
the common point for all grounds (equipment chassis, utility ground tie in,
telephone, etc.) inside the shack. I use 4 ground rods along a line with 8'
spacing, tied together with copper strap along one side of the shack. A
complete "perimeter ground" would be even better. At this point you should
have protectors on all your feed lines and control cables. It is also a good
idea to do the same thing at the base of each tower, and to ground the
shield of your feed lines to the base of the tower.
I do not know what the soil conductivity is in your area. I am in a very
low conductivity limestone rock and caliche soil area and on a ridge where
we get lots of electrical activity during spring and summer storms. The key
is to allow the pulse from the strike to spread out quickly over the area. I
use copper strap rather than wire for grounding. It has much less
inductance than wire and will impede the energy less. I have a ground strap
on each tower leg connected via a strip of stainless steel between the
galvanized steel and copper surfaces. Each strap leads to an 8' ground rod
mounted just outside the concrete base. Each of these three rods then has
three 30' copper straps, and each strap has 3 ground rods attached to it.
There is no "ring ground" or perimeter connection, and in my opinion this is
a waste of wire which should be used to add more radials right next to the
tower. All of these connections are made with Polyphaser compression clamps.
Cadweld would be even better.
IMHO the idea that the taller tower will "protect" the shorter when is not
anything to bet on, even if they are closer than the 80' rule apart. I have
seen lightning hit a car parked 20' from an 80' high tower and not the
tower. Who knows why?
I would give the concrete a full 30 days to cure before erecting the towers
73 John N5CQ
[mailto:email@example.com]On Behalf Of firstname.lastname@example.org
Sent: Wednesday, August 11, 1999 11:24 PM
Subject: [TowerTalk] Tower Grounding Query!
Got some questions that i would like to hear opinions on.
Building two towers-one will be 55 ft. high including a short mast with
tribander on it. Under it i expect to have a pretty good ground system. The
tower will be about 65 ft. from the back of the house, in the center of the
backyard.The other tower will be a shorter housebracketed tower at the back
of the house. .....
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