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Author Topic: Moxon and lightning safety?  (Read 264 times)
KC9KEP
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Posts: 208


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« on: September 20, 2009, 06:06:22 PM »

Hello all!

I've got plans to build the Moxon antenna as featured
in QST magazine (for 20 meters).

I've purchased a 10-foot tripod, 10-foot mast and rotator.
The ridge of my roof is about 21 feet high, so I hope to achieve
a height of 37 feet.
I plan to ground the tripod via aluminum wire
and ground rod.

Here’s the problem: my wife is concerned about lightning.
Truth be known, the risk bothers me too.

Would I be better off with a tower?
Where I grew up, everyone in my parents neighborhood
had TV antennas on their roofs and I don't recall any
one's house getting struck by lightning.

There are a few cotton-wood trees in my yard that
would make taller targets than my tripod & Moxon.

How about a lighting rod at the top of the tripod?
I've never seen anyone do that?

Thank you!

--Tom Nickel
KC9KEP
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KB9CRY
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« Reply #1 on: September 20, 2009, 07:16:32 PM »

Why not so a search on the main forums page for Grounding and do your own homework.

There are 5 elements of a lightning grounding system, regardless of the antenna design.  You need all 5.
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K6AER
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« Reply #2 on: September 20, 2009, 07:47:44 PM »

There are many sites devoted to what is necessary for lightning protection. Read the tutorials at PolyPhaser, Hager, ARRL and other lightning information sites.

Lightening does not favor one type of tower over another with or with out an antenna. The discharge path is ionized between the cloud and ground and may travel many miles before the discharge is made. The earth conductivity and its location to the discharge cloud set up the discharge location. If a tower is located near by and has good conductivity the tower will take the discharge but in the end the lightning strike was going to hit near by with or with out ham intervention.

The key to equipment surviving a lightning discharge on the antenna is to keep the discharge from entering the ham shack. Only proper grounding techniques and complete grounding system will mitigate any possible damage.

A while back I posted a reader digest version what has to be done to reduce lightning strike damage. You can read it at:

http://www.eham.net/articles/13461
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KC9KEP
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« Reply #3 on: September 21, 2009, 03:14:33 AM »

Thanks for the information.

It didn't occur to me to search for "grounding" so that
information is helpful to me :-)

I've got an ARRL antenna book and it devotes about
2 paragraphs to the topic, so it's not much to go on.

Thanks again,

--KC9KEP
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AC5UP
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Posts: 3845




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« Reply #4 on: September 21, 2009, 03:24:01 AM »

A few years back a tree approximately 200' feet from my house took a stroke from Thor's hammer. Damn, was that loud... Neighbor lost almost everything electrical in their house. I had two phones, a modem and other assorted goodies go toes up. Fortunately nothing high-dollar was damaged.

Point being that it doesn't take a direct hit to toast your muffins.
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WA3SKN
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« Reply #5 on: September 21, 2009, 12:17:51 PM »

Interestingly, "grounding" and the reasons for grounding have changed over the years.  But lightning protection is still one of them!
Both the Polyphaser and I.C.E. websites have some good reading and would be the first place to start.  Also read up on "single-point grounding", as you will want to connect all your grounds together.
73s.

-Mike.
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WB5JEO
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« Reply #6 on: September 21, 2009, 12:33:03 PM »

When you study the referenced pages of information on station grounding for lightning protection, you'll see that it's a not insignificant undertaking. I do it, but I consider it mitigation, rather than protection. I may well take a direct or very close hit with no damage at all. Or I may take some damage. The ways of high voltage electricity are strange and wonderful. You can find every imaginable kind of tale about no damage from a hit on an essentially unprotected station to textbook set-ups that take hits routinely with no damage, to stations with what look like good systems that took heavy damage. None of them should be used to make decisions. As you observed, the chances of most of us being hit are slim, and there's no "typical" lightning event. I put in protection because I don't want to deal with the aftermath of that unlikely event, but I also buy insurance so that my response to a disaster is more time than money.
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K9KJM
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« Reply #7 on: September 21, 2009, 11:30:38 PM »

To protect your home from direct lightning damage, All you need to do is properly GROUND your tripod with a fairly heavy sized conductor, With no sharp bends to a good ground system.
(#6 is considered a minimum size wire, Flat copper or aluminum strap several inches wide is much better, Hollow copper tube can also work well.)

 
(To properly protect all your equipment connected to the antenna is a little more complicated.)

For some good info:

http://members.cox.net/pc-usa/station/ground0.htm

(Lightning DOES strike ungrounded TV antennas on tripods and cause damage to a home. A local home near here had a strike that missed a childs crib by only a few feet as the energy from the tripod strike went to ground THROUGH the house!  
Having a properly grounded tripod on the roof (Or tower next to the house) Makes you much safer than not having the grounded system.)


(Taller trees in the area is NO insurance of not being struck)
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