A lot of guys out here (and elsewhere) have a vertical (VHF and/or UHF) at
the very top of their antenna stack. I'm pretty sure this qualifies as a
pointy device. When I installed my first tower out here I recall reading in
a then-current ARRL Antenna Book that it is much better to have a horizontal
antenna at the top. Mine will be a 2-meter Yagi (horizontal) but the mast
will still stick up a bit above it to support the boom truss. Guess I could
make that part out of fiberglass.
To show you the difference location can make, I have one friend, K0TY, in
South Dakota who operates a repeater and has been hit two (or was it three?)
times in maybe ten years. Out here (SF Bay area) of all the guys I know
with pointy things (vertically polarized antennas) on top of their towers,
no one has ever been hit even once.
I don't know about porcupines, but plastic owls sure are a waste of money!
73 & Happy July 4th - JC, K0HPS
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com]On Behalf Of Gary Schafer
Sent: Monday, July 03, 2006 10:27 PM
To: 'Nick Pair'; 'Christopher J Galbraith'; towertalk@contesting.com
Subject: Re: [TowerTalk] Grounding base slab and pier tower bases
If you want to minimize the chance of a lightning strike to your tower do
not place a device on the top with sharp points. A blunt point rod should be
placed at the top.
The so called lightning preventers or porcupine items do nothing to abate a
strike. In fact they can increase the chance of a lightning strike by
providing sharp points where early streamers can form. There is no way to
"bleed off" any energy from a tower as the earth can re-supply it many times
faster than you can get rid of it.
The reason some of those manufacturers boast of less lightning damage with
their systems installed is that, if you will notice, they all recommend a
very good ground system along with them. Ever wonder why?
There are people that have installed those "dissipaters" that report less
lightning damage after their installation. Ever wonder why?
It is because they have improved their ground system!
73
Gary K4FMX
> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Nick Pair
> Sent: Monday, July 03, 2006 12:44 PM
> To: Christopher J Galbraith; towertalk@contesting.com
> Subject: Re: [TowerTalk] Grounding base slab and pier tower bases
>
> Hello Chris,
> Yes I could with the cravat that you MUST provide a low impedance ground
> path to building ground and a lightning ground system.This path must be
> direct as possible and all bends radiused with at least a 6" radius. Roof
> installations are particularly hard to protect without a much larger
> expenditure for funds in copper! Roof cell sites are usually fed with a
> 4/0 cu wire to a ground grid at earth level. That being said you have to
> live with what you have sometimes.
> With a reasonable ground a device similar to the Nott Ltd. GS-1
> (nottltd.com/lightning .html), Ericson Broadcast Sales SC-1 or SC-3(or
> several SC-3's)
> (ebsradio.com/lightning.html) or home built like here:porcupine.w4zt.com
> will reduce your strike chance by 95% or so. The last 5% is due to the
> fact that sometimes the charge builds up faster than can be bled off with
> the discharge array you have. I have also built devices by using 3/8 to
> 1/2 inch stainless steel cable one foot long in a compression one hole lug
> connector (or crimp if you have access to large enough crimper) on a U-
> bolt with galvanized or ss saddle on the tower leg. You fray out the cable
> to create the largest area of points you can. Which ever one you choose,
> try to place it higher than all antennae. If you must use a rotating
> member to do this , use a #6 or better insulated wire to the device and to
> the nearest tower leg with minimum for rotation. Similar metal clamps
> always.
> Of course YMMV
>
> Good Luck,
> Nick WB7PEK
> Christopher J Galbraith <cgalbrai@umich.edu> wrote:
> Hi Nick,
>
> Could you recommend a particular lightning discharge device? The
> application is a 40 ft tower on top of a 50 ft building in a relatively
> lightning prone area (SE Michigan).
>
> Thanks and 73,
>
> Chris KA8WFC
>
>
> -----Original Message-----
>
> (snip)
>
> If you lived in a more lightning prone area I'd recommend a lightning
> discharge device for the tower top. They do a good job of keeping the
> tower
> potential below where a leader arc can be created preventing a strike from
> occurring to the tower and a radius equal to the height around it.
>
> Nick WB7PEK
>
>
>
>
>
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