This subject has raised a lot of interest lately, and I am deeply
involved, so here's a bunch of words for those who are interested. Feel
free to delete!
Before you homebrew, consider ---
There some basics with these devices that are important:
1. Soldering anything to do with lightning protection is worthless - it
vaporizes with the first tickle. Mechanical clamps, machine crimps, or
"Cadweld" type assembly will work.
2. Standard galvanizing is only slightly better -- that's why towers are
hot dipped.
Heavy wall aluminum tube conduit is not bad, stainless steel is best.
Regular electrical conduit (galvanized steel) is not good. Galvanized
steel wire is good only till the sharp cut point rusts. Bronze or brass
wire is better.
3. The more sharp points the better, but very fine wire can burn up over
time.
4. A good DC path to ground is essential, so dissimilar metal junctions
can defeat the whole idea if not properly protected. Corrosion leads to
elevated resistance, then to arcing and finally, destruction.
It is very important to understand that the dissipation principle is
effective only if properly employed. One crappy little brush in a
lightning storm could do much more harm than good by instigating a flare
that provides an ionized path for a bolt to follow, whereas a sufficient
number would have wiped out the risk. One must consider HIS risk with
HIS installation in HIS geographic location and act accordingly. It would
be a bad mistake to put up a little home brew device to "test the water"
with a plan to add more later, "just in case." It is also a bad mistake
to make a homebrew that will change in time due to corrosion, working
loose, or the like. Nothing against homebrew done right, however. Years
ago we cut hundreds of 6 inch SS spikes from surplus CB whips, spun 'em
against a fine grinder to needle point, stuffed the butts into 1 inch
holes in a piece of Aluminum bar stock and peened them in to stay with
hammer and punch. Bolted it to the top plate at the bottom of a G7 and,
after losing 3 in previous years, lost nothing for years thereafter.
We've inspected brushes that have turned blue and had a few inches burned
off
the ends, and some that had no brush left at all! In such cases, more
brushes would have saved the loss, but at least the hit was avoided.
Quantity and sizes required require either professional advice or a well
educated guess.
The example I offered the other day of the main street bridge in Jax is a
good case of doing it right. I've not seen a strike count, but will bet
they are rare.
The numbered points at the beginning of these comments might seem to
indicate the need for heavy current carrying capability, which may be
misleading. The current carried in a dissipation system's path to ground
is not expected to be able to handle a strike. The absolute low
resistance continuity of the path is the critical element. If you recall
grandfather's lightning rod system, the conductor to ground was a loose
mesh, finger size copper or copper clad steel wire, a highly inductive
and therefore lossy line that helps with the dissipation process.
When we do our demo, I produce a 2-4 inch spark from globe to ball. I
next use a single sharp point, and produce only a weak 1/4 inch spark,
and then I follow with a 700 point "porcupine," as we dub it, and get
nothing at all, and one can't even feel the static ruffle of the hair on
his arm. A lead to ground from the brush instead of just my body will
produce a barely visible 1/64 inch spark. My body is the usual ground in
the show, and although we throw 20 to 30K volt bolts, the current is only
enough to cause a twitch. In a protection system, the cumulative current
from several hundred or even a dozen "Porkies) would require
proportionally larger current carrying capability in the conductor that
serves the multiple units. Just like all of the creeks becoming a river.
These dissipators are supplied in many forms: Brushes with tens,
hundreds, and thousands of wires of 6 inch to 6 foot length of various
wire sizes from .003 to .100 inch and more. Commercial units are usually
SS, old fashioned lightning rods were copper, bronze, even galvanized
steel. Long runs of brush that looks like an endless bottle brush only
larger run along parapets and superstructure (The bridge at Jax has
both). Marine use is growing rapidly.
If you decide to buy some of these devices, AVOID marine supply houses
unless you've won the lottery. The prices run 2 to 5 times higher than
anywhere else. "If you own a boat you must be rich," seems to be the
thesis!
Here I will insert some of the text we supply in our display, for a bit
more background:
"A dissipator provides an ionization discharge path at each stress point
on a
communications tower or antenna installation. By discharging these points
into
the surrounding air, the possibility of a lightning strike is greatly
reduced. There
is an additional advantage: with the reduction of static charge buildup,
there is a
reduction of noise heard in the receiver.
The stress points are the highest and outermost points on the tower or
antenna
installation. Ideally, one or more dissipators should be placed at the
top of the
tower, top of the mast, and ends of booms. None should be affixed to any
radiating or active element of the antenna.
Low voltage dissipation functions by the conduction of a small Direct
Current
(DC) to or from ground. Charged particles in the immediate atmosphere
surrounding the dissipator are neutralized by the opposite polarity on
the
charges present at the tip of each wire in the dissipator. The voltage
buildup in
the area is thus retarded and strike potential is less likely to be
reached.
Obviously a continuous DC path to ground must be provided. Normally, the
tower or mast metal is adequate for this discharge, provided that they
are
properly connected to an adequate ground. At least one 8 foot copper
covered
ground rod is required for each leg of a tower, connected with a solid
copper
(low inductance) conductor (no less than 4 AWG) to each leg. Copper bar
stock
(1/4 X 1") is better for high risk areas. Connections must be made with
positive,
low corrosion clamps or a welding process like "Cadweld." The rods should
all
be connected with a circle of the heavy copper wire; consider 3 or more
additional rods, each 6 feet from the other, connected as before, in high
risk
areas such as central Florida. This sounds like over-kill for a low
current DC
path, and it is, but it is also there for that big strike that nails us,
no matter what
we do to avoid it. A direct strike can be 18,000 amps, so, the more
current that
your system can carry to ground, the less remains to plow through all of
your
property.
Remember Grandfather's house and barn, with the lightning rods and
weather
vanes? They were and still are the best protection for the money a rural
or any
home at risk can have. They were usually connected to each other and
ground
by a loose-weave copper or copper-clad steel stranded cable. This highly
inductive path was perfect for charge dissipation, if not for a heavy
strike."
I hope this is of some value in the dissipator discussion. Please don't
hesitate to hit me for whatever else we can contribute, or for details on
units that are available. Yes, we sell them, and are getting together
economical kits, too.
73
::Press Jones, N8UG
::The Wireman, Inc.
::Landrum, SC 29356
::use n8ug@juno.com or (864) 895-4195 for tech help
::orders only use 800-727-WIRE(9473) or cqwire@juno.com
The following is left as a reference as to what sparked my treatise - I
consider it to be a way to do it all wrong - sorry - Press
<<<<On Thu, 07 Aug 1997 23:27:10 -0400 Martin Gary
<n3hrt@eagle1.eaglenet.com> writes:
>I'm not sure of the exact conductor size, its about the size of your
>index finger. There was an article in QST (or 73) that discribed the
>way
>you make these "spline balls."
>
>Go to Lowes and get some of that galvanized fence wire, cut it into
>about 2 ft. lengths. Make sure that you cut the wire at an angle, so
>that its pointed on the end. Then take a piece of 1 inch x 4 or 5 ft.
>electric conduit, cut some vertical slices into one end and spread
>them
>apart slightly. Then jam as many peices of the fence wire in it as you
>can. You only need to put the fence wire in a couple of inches. Clamp
>the wire in the conduit with a stainless steel hose clamp. Then take
>your handy-dandy torch and some good flux and solder the whole thing.
>
>Now if you spreed the wires out they will form a ball. It ends up
>looking kind of strange. Make sure you bend the conduit at the bottom
>(where you would attach it to the tower). The bend should be about 35
>degrees.. so that when attached it will stick out from the tower.
>Connect the "spline balls" to the conductor mentioned above and thats
>it.
>
>I hope this is clear enough... If its not let me know!
>73 de Martin - N3HRT.
>
>
>Russ Ellsworth wrote:
>>
>> Martin Gary wrote:
>> >
>> > Actually they do work, the power company here in Southern Maryland
>put
>> > them on all
>> > of their communications towers and they haven't been hit since.
>> > To get them to work properly you HAVE to have a GOOD ground. You
>also
>> > have to have
>> > a BIG conductor connected directly to them and to the ground
>system
>> > below.
>> > You also have to have a lot of points. Several hundred or so.
>There has
>> > to be
>> > enough surface area to let the charge bleed off before the strike.
>>
>> snip
>>
>> Martin, Do you know how big the conductors are the power company is
>> using or are they relying on the metallic towers (if they are
>metallic)?
>> These "spikey things" may be worth the effort for us Hams to install
>if
>> the professionals feel they are.
>>
>> Russ Ellsworth WA6CWV, Boise Idaho
>
>--
>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
>
--
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
|