[Towertalk] Lighting Protection and Roof Towers

Thu, 12 Dec 2002 11:31:38 -0600

One more response:  I still have a Hy Gain vertical that survived a 
direct hit.  I looked out the window at the right moment and saw it (and 
heard it).  The feedline from it was terminated outside the house at the 
time, not connected to any of my equipment.  I hadn't installed my 
current lightning protection system.

The average lightning strike peak current is about 18,000 amperes.  Most 
are less and some are far larger, to arrive at that average.  90% of 
lightning strikes consist of a few strokes of microsecond duration, 
separated by milleseconds.  About 10% of strikes maintain current of a 
few hundred amperes between the individual strokes.  This 10% is 
especially damaging, but they haven't set the Empire State Building on 
fire, yet.  Read Uman's books for more information about the 
characteristics of lightning strikes.

Yep, it's a gamble.  We can't guarantee 100% protection, but we can 
certainly reduce the odds against us, even when our equipment and 
buildings take a direct hit.

73 de WOŘW

Jon Ogden wrote:

>on 12/11/02 6:09 PM, Jason Hissong at jhisson1@columbus.rr.com wrote:
>
>  
>
>>I have been reading Polyphasor's site about lightning protection.  And it
>>seems that a roof tower is probably the worst type of mounting you can have
>>(great!).  However, it mentions what can be done about it.
>>    
>>
>
>It's not ideal, but it can be worked around.  Just make sure you don't have
>any sharp bends.  I made this mistake and while I haven't been hit, it's
>best not to have any.  A sharp bend looks like a highly inductive element in
>the wire.
>
>Realize too that one aspect of lightning protection is that lightning will
>generally hit objects that are the highest in the immediate area.  A roof
>tower, may still be significantly lower than other items in your immediate
>area such as trees, etc.  YMMV, and it's not something you want to count on,
>but being lower than other objects around you does mitigate some risk.
>
>  
>
>>It seems that the best thing to do is have multiple connections to a ground
>>perimeter around the house.  My planned installation is to ground each leg
>>of the GM roof tower with #2 solid copper wire to 4 ground rods
>>interconnected to each other.  Polyphasor recommends using copper straps
>>(which I would be willing to do).  Where would one get some locally
>>(hardware stores?)
>>
>>    
>>
>
>The thing that kills lightning protection is inductance in wire.  Lightning
>will always take the least inductive path to ground.  Surface area of the
>conductor carrying the lightning current is what will reduce inductance.  I
>grounded my roof tower with flat braided cable.  It's roughly akin to taking
>the center conductor and dielectric out of a piece of coax, but it's larger
>and it is also solder plated.  It holds up very well in weather.  It is
>available from Cable-Experts but it's expensive.  About $1.00 a foot if I
>remember correctly.
>
>Connecting the ground rods can be done with #2 or #4 copper wire.  Yes, Poly
>recommends strap, but that is very costly, and you have to weigh the
>benefits of the extra work and cost of strap over say #4 wire.  A lot of
>guys will say that you should only use strap, but others say #4 wire is
>fine.
>
>Ground rods have an effective "sink" area in the earth of a radius equal to
>their length.  So an 8 foot ground rod will cover a circular area in an 8
>foot radius around it.  So therefore, you need to put ground rods every 16
>feet and interconnect them.
>
>
>  
>
>>My primary concern is not my equipment (although it would be nice) but my
>>home.  The tower is going on my garage.  My lot is surrounded by trees and I
>>have electrical poles about 40ft behind my house.  I would be unable to have
>>a ground perimeter around the house due to the fact that the ground strap
>>would have to go over things like a driveway and a porch.  Any ideas on how
>>to get around that?  (The strap would be running over the top of the
>>driveway and porch).  I hear you want to bury the copper strap and the
>>ground rods.  I would be able to basically make a "U" around the house.  The
>>shack ground would be connected to the perimeter ground.
>>    
>>
>
>While a perimeter ground is great, it isn't absolutely necessary.  I have a
>similar problem and my perimeter ground runs around most of my house and
>garage.  You want to create as large a sink for the lightning as possible
>and you want to try to pull the potential of your tower down to earth as
>much as possible.
>
>Half the battle in lightning protection is just that.  You want to prevent
>static from building up on the tower or antenna, etc.
>
>  
>
>>Anyways... maybe I am being overly worried.  I live in Columbus, OH which I
>>would consider a moderate frequency for lightning.  Of course, it only takes
>>one.....
>>    
>>
>
>No, lightning is a very powerful and scary force.  You need to have respect
>which you do.  But also realize that there is only so much you can do with a
>given budget and that there is no situation that will 100% protect 100% of
>the time.  You want to take you statistical chance of a strike as low as
>possible.
>
>  
>
>>Initially, the four legs of the tower will be grounded when I install the
>>tower in the next couple of weeks.  Over the winter, I will add additional
>>ground rods around the house and connect them together.  So by spring, I
>>hope to have enough protection.  What type of compound should I buy to make
>>sure the corrosion is minimized where the copper strap/wire and ground rods
>>meet?
>>
>>    
>>
>This sounds fine.  If you are using copper wire and copper ground rods, you
>don't need to worry about any compound.  The places where compound is needed
>is when you connect dissimilar metals.  Like if you connect a copper ground
>clamp to your galvanized or aluminum tower.  That compound is called Noalox
>and it is available in the electrical section at Home Depot or probably just
>about anywhere like that.  It might not hurt to use Noalox on a
>copper-copper connection, but I just don't think it buys you anything.
>
>  
>
>>I guess if there was a direct strike I am screwed anyway :)
>>    
>>
>
>Yup.  And life's too short to worry about it anyhow!  :-)
>
>I've had my roof tower up for 5 years now and never a problem.  I live near
>Chicago and we get wicked thunderstorms.
>
>73,
>
>Jon
>NA9D
>
>-------------------------------------
>Jon Ogden
>NA9D (ex: KE9NA)
>
>Citizen of the People's Democratic Republik of Illinois
>
>Life Member: ARRL, NRA
>Member:  AMSAT, DXCC
>
>http://www.qsl.net/na9d
>
>"A life lived in fear is a life half lived."
>
>
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>  
>

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