He's not there if it's a single run. 75' is considered the maximum for
effective lengths. (see links in previous post) With the soil and rock
just 2 to 3 feet down he needs a "network", based on 3 radials. One for
each tower leg with interconnecting wires forming a ground screen. The
more, the better within budgetary limits.
The rest of your post has some good information.
I've found little to no information the the ARRL handbooks about safety
grounding and nothing on UFER safety grounds.
Contrary to one post, there is little danger of blowing up a concrete
UFER ground.
The concrete has an abundance of free ions and eventually will dope the
surrounding soil. How much? I don't know.
This is simply a safety ground (earth) and has little to no effect on
antenna operation.
73
Roger (K8RI)
On 5/4/2016 Wednesday 12:17 AM, Hans Hammarquist via TowerTalk wrote:
Hi Dave,
I believe you are pretty much there. The 120' #2 wire serves as a rather good
ground for the tower and also serves the function as the equalizer (see #2).
In my opinion there are two main things you want to accomplish protecting
yourself from lightning strikes:
1) You want to discharge as much as possible of the current from the lightning. That you
do with grounding rods, grounding wires, grounding plates what have you spread around and
connected to your tower. )In my case I use 9 rods connected with #4 wire to my tower). I
guess your 120' wire is good and, needless to say, more wires laid out is a star pattern
around your tower will help. What I understand, from reading, it is more important to
have more wires and not too long once. I would guess, and this is not based on anything
but my guts feeling, lengths of 20 - 30 feet would be optimal. Yes, the more the better,
NOT: "The longer the better" NO!
2) You want the voltage difference between your tower and your equipment to be as small
as possible. That's where the #2 cable between your tower and your shack comes in. Here
should be:" The more and the heavier the better". I believe several wires in
parallel and spread apart a few inches to a few feet is good. (Similar effect as a Litz
wire.) The idea is that in the event of a lightning strike you want the shack to assume
the same (or as close as possible) voltage as the tower thereby reduce voltage induced
current in your feed and control wires. (I have a short, 6 inch wide, aluminum band
between my shack and my tower. The tower is only five feet away from my shack so that was
simple.)
Good luck and wishes of no lightning destruction in your tower or shack.
best 73 de,
Hans - N2JFS
-----Original Message-----
From: David Merchant <merchdl@gmail.com>
To: towertalk <towertalk@contesting.com>
Sent: Sun, May 1, 2016 1:11 pm
Subject: [TowerTalk] Ledge Lightning Ground Advice
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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
--
73
Roger (K8RI)
---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|