Ed, I am replying to your questions on the reflector so that all
subscribers can share in the discussion and further contribute.
Mark
>Edward Soriano, M.D., DU1OZ, KB3CNO Cavite City, The Philippines, wrote:
>I just erected my first 40 footer crank-up tower last Saturday. It is
>on top of a 12 feet concrete post. It was designed in such a way so
>that I can tilt the tower above an adjacent roof.
>Questions:
>1. What is the best way to ground tower on top of structures like
>buildings and in my case a concrete post? I have one 5/8 copper rod
>embedded 9 feet below the ground besides the tower. My plan is to
>connect it to the tower with a no.4 wire using eye terminals.
The consensus seems to be to connect ground straps to each of the three
tower legs and run them down like radials to at *least* three ground rods
adjacent to the tower base. In your case, these straps will just be 12
feet longer because of your elevated base.
>2. What do you mean exactly by multiple rods tied together? Got plenty
>of no.8 stranded wires in my garage. Will this suffice?
Again, the idea is to collect and/or dissipate charge from/to the earth. A
single ground rod alone cannot do this effectively. One rod saturates an
area around it radially out to a distance about equal to its depth.
Therefore, adjacent, 8 foot rods should be spaced 16 feet apart. Three
rods spaced to form an equilateral triangle, 16 feet on a side, around
your base would be a minimum start. The rods should also be connected to
each other in a ring with straps that would trace the circumference of
this triangle. This makes an effective connection to a *much* larger
volume of earth. For best performance, add 3 more rods for a total of 6
rods. Number 8 gauge is a little small. For my tower installation, I
plan to use 3/4 common copper water pipe for my ground rods and straps
(HQ to the rescue!) to make most efficient use of the copper, accounting
for skin effect. I would connect the ends by flattening them, drilling
holes, and joining with a copper split bolt, to keep all similar metals. A
joint compound with copper particles would be ideal between mating
surfaces. I think there is an anti-seize paste for threaded fasteners
named KoprCoat or something like that that is filled with copper particles
that could be used for this (could someone please help out with this
part?). This way there are no dissimilar metals, promoting greatest
longetivity for the joints. I'm not sure that just soldering them
together is good enough for underground service.
Ed, Let me try some ASCII ART (view or print this with a monospaced font)
to illustrate:
o
|
|
|
o
/|\
/ | \
/ T \
/ / \ \
/ / \ \
|/ \|
s-------o-----------o
/ \
/ \
/ \
o o
This is not quite to scale, of course. "T" is your tower base. "o" is a
ground rod. The radials, tied to the tower legs, spread outward, with 8
foot deep ground rods connected every 16 feet. The first set of rods is
connected together to form a triangle. There should also be a ground strap
from this system to the ground system for your home (service entrance,
"s").
>3. Is there a need to run a separate wire from the lightning rod to the
>grounding rod? Or can I just ground the lightning rod to the tower?
Not if there is good contact all the way through the tower legs. For a
crankup/foldover, you may need a jumper across pivot joints or rollers, as
these might not be reliable conductive paths like bolted leg joints. Some
companies insist on a separate copper conductor running up the tower,
which is the best way to absolutely assure a good path, but I don't think
this will be necessary.
>Will stainless steel be good as a lightning rod? You see I live very
>near Manila Bay.
I would think so. Put a cone point on the top and bolt it securely to the
tower, preferably at a point above your antennas.
>4. Is there a need to have a separate grounding rod for the tower,
>lightning rod and coaxial lightning arresters? Or can I connect
>everything to one single ground rod. If the latter is the case is there
>no chance lightning will still find a way back into my shack from the
>ground through the single ground rod?
All grounds are tied together, as you can see from the "art". Multiple
rods are what you are counting on to dissipate the strike current without
making it want to 'search' elsewhere for a ground path.
Ed, I hope this helps!
--...MARK_N1LO...--
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|