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Author Topic: Dipole And Lightning Protection Strategy?  (Read 5943 times)
TPELLE
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Posts: 18




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« on: August 07, 2012, 04:12:21 PM »

I'm not licensed yet, but studying for the Technician test. I am thinking about what sort of rig and antenna to set up though, and am planning a 40 meter dipole strung between two trees perfectly located about 120 feet apart in my back yard, fed with ladder line.

A major concern is lightning protection. I did a search of this site and got 37 PAGES of hits! Read many of them until my head felt like it had suffered a lightning strike!

So here are my thoughts so far, and I'm hoping for some constructive criticism.

First, the dipole will be strung between the trees with a rope connection to each tree. One connection will be solidly cleated off, while the other end will be rigged through a pulley with a counterweight to allow for the wind moving the trees around. The feed connection will be in the center, of course, and will hang down to a short, maybe 8 foot tall post about 10 or 20 feet or so from the house. I will connect the antenna side of the feed line to the center poles of a knife switch. The bottom two connections of the knife switch will both tie to a heavy ground cable which will be bonded to a driven ground. The top poles of the knife switch will terminate to a banana plug assembly, which will permit the house segment of the feedline to be unplugged and pulled back to the house well clear of the antenna proper.

The physical attachment of the house side of the feed line will be via a set of hooks over which the "last" ladder rung can be securely hooked, but remain easily detachable.

I would also use the typical spark plug spark gap static discharge assembly also attached to the center poles of the knife switch.

My major concern is the viability of the knife switch/banana plug arrangement, and how that would play well at 7000 kHz.

I recently had a demonstration of the destructive power of lightning, as a 50 foot ash tree in my front yard was peeled like a banana and the trunk split into 5 pieces by a lightning strike. My design described above will give me the ability to disconnect the feed line from the rest of the antenna and physically separate it, thereby providing a hopefully adequate isolation gap from the main part of the antenna.

I have no illusions that the knife switch alone will be adequate to disconnect the feed line segment going to the house from the segment leading to the dipole. With enough voltage to jump thousands of feet, once it encounters the little gap of an inch or so on the knife switch, the lightning bolt will just sneer!

Any thoughts or suggestions?

« Last Edit: August 07, 2012, 04:23:48 PM by TPELLE » Logged
W9GB
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Posts: 2659




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« Reply #1 on: August 07, 2012, 05:05:36 PM »

Lightning will find a path ... this electrostatic discharge usually seeks
 the easiest or most conductive -- of the options offered.
http://en.wikipedia.org/wiki/Lightning

Electrical lines, telephone lines, antennas and their transmission lines,
or copper pipes are all potential pathways for lightning.

Polyphaser
http://www.protectiongroup.com/PolyPhaser

and Harger are two providers of lightning protection products.
http://www.harger.com/
« Last Edit: August 07, 2012, 05:14:28 PM by W9GB » Logged
K4SAV
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Posts: 1851




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« Reply #2 on: August 07, 2012, 06:27:19 PM »

I hate to burst your bubble, but this system has zero protection from a direct strike.  Ladderline will simple evaporate when hit by lightning, maybe the antenna wire as well.  When that happens lightning will go wherever it finds a good ground.  Also a single ground rod is one notch above useless for sinking lightning currents.

Where are you located?

Jerry, K4SAV
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KE3WD
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Posts: 5689




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« Reply #3 on: August 07, 2012, 06:58:49 PM »

The link already given above for Polyphaser. 

Study what their website has to say, communicate with them if unsure what to buy and how to properly install Polyphaser protection and enjoy a very modern solution that flat WORKS. 

73
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N4NYY
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Posts: 4820




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« Reply #4 on: August 07, 2012, 07:59:03 PM »

In addition to the above comments, go to W8JI website and look up the lightning protection.

Bond your ground rods outside the house. Use a single point entrance panel where your cables enter the house.
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W6EM
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Posts: 900




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« Reply #5 on: August 07, 2012, 08:33:26 PM »

In addition to the above comments, go to W8JI website and look up the lightning protection.

Bond your ground rods outside the house. Use a single point entrance panel where your cables enter the house.

Good suggestion.  JI has some good info.  The key to good protection is what is called an equipotential plane.  Small coax widgets won't handle 10s of thousands of amps and joules from a direct strike.  Frankly, most anything we can afford these days in terms of copper wire or cable will fuse or burn open too, if in the path of a direct strike.

Bond all ground rods to your house grounding electrode (usually a rod, but can be other things).  make sure all shack equipment and feedlines terminate in something like JI uses.  A copper plate that is tied to your ground rod/electrode system.  The key is to make sure all gear rises (yes, rises above ground) to the same voltage to ground if a direct strike induces enough current to create a voltage rise.  And, for your safety, that there is no difference of potential beween supposedly grounded equipment in your hamshack.  Disconnect coaxes/feedlines from equipment if you can.

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KA4POL
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Posts: 2125




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« Reply #6 on: August 07, 2012, 10:18:34 PM »

It is good you had a demonstration of the power of lightning, sorry for the tree. Many don't have an idea what can be done by lightning. If you can get your hands on the 2002 issues of QST for June, July and August you'll find a series about protection from lightning. Also the ARRL Handbook as well as the Antenna book are helpful resources (not only for lightning protection issues).
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N4NYY
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Posts: 4820




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« Reply #7 on: August 08, 2012, 05:52:29 AM »

I got a single point entry panel from this ham. Thanks to eham members that pointed him out.

http://www.kf7p.com/KF7P/EntrancePanels.html
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W5FYI
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Posts: 1046




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« Reply #8 on: August 08, 2012, 07:56:18 AM »

I would not waste the effort of putting the knife switch at the antenna side of the transmission line--I can't see that it will do much good there.  What I would do is connect the ladder line to the antenna (good planning to use the pulley). On the shack side of the transmission line I would have the spark-gap connected to ground rod, as you plan, and would run the line through a metal bulkhead that is also connected to the ground rod.  I would then mount the knife switch on the inside of the metal bulkhead, within easy reach for disconnection when you are not using your radios. The switch's bottom terminals can go to your radio or tuner, the top terminals to the bulkhead (or, for fun, to a neon bulb, so you can watch the flashes of static charges).  The knife switch location inside the shack will be a lot more convenient, and the metal bulkhead will do much to keep direct lightning strikes outside the house.

There were plans for building a spark plug spark gap in older ARRL handbooks and antenna books. Depending on the distance between electrodes, the gap would be safe for powers exceeding a kilowatt. Polyphaser or Alpha-Delta lightning arrestors in the coax between the tuner and rig, properly grounded to the entry bulkhead, will further protect your radio. GL
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K4SAV
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Posts: 1851




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« Reply #9 on: August 08, 2012, 08:10:32 AM »

When I lived in Florida (for 38 years) I saw lots of lightning damage, and I have seen the exact same system you described except there was a relay instead of the knife switch.  I have also seen the results after it was hit by a direct strike (also in Florida).  It wasn't pretty. I will describe what happened for the amusement of the readers, and maybe a little education for some of them.

If you look thru the large volume of information on lightning protection for a method to protect a dipole, you probably won't find anything (with the exception of something I wrote).  Most just ignore the problem because the solution isn't something most can, or are willing to implement.

There are many who have poor lightning protection systems and sustain minor damage from a strike.   What they may have experienced is a small secondary finger off the main stroke, and the main stroke went somewhere else.

There are many hams that think you can protect an antenna and associated equipment from lightning when the main lightning path is just a feedline.  They always give advice like: "buy a surge suppressor and put it on the line";  "that works for me, I have never had a problem."  - Probably because they have never had a lightning strike (which they always fail to mention).  They also continue to repeat the many lightning myths they hear, which serves to totally confuse most hams about the proper method of implementing lightning protection. They should pay special attention to this description.

The strike hit the antenna wire somewhere close to the middle.  It blew the ropes that were holding the antenna to two trees and the antenna fell to the ground.  The trees were not damaged.  The main strike did not go thru the antenna wire but there was enough current to turn the #12 solid copperweld wire into the consistency of a limp noodle, which would break very easily.  The ladderline exploded, blowing bits of plastic all over the yard in little confetti like pieces, most 1 to 2 inches long.  It exploded so quickly that the plastic didn't even have melt marks.  There was no copper left.  It all evaporated.  The box containing the relay exploded.  That box was located on a 5 ft post, 10 to 15 ft from the house and there was a ground rod right under it.  The stroke ignored the one ground rod.  After the ladderline disappeared, the stroke split into several fingers attaching themselves to the house chimney, spigots, external AC outlets, and metal roof vents.  That was verified by looking at burn marks.  The largest part of the stroke went into the rebar that was located inside a concrete block at a corner of the house.  It blew a 6 to 8 inch diameter hole thru that block.  It traveled thru the concrete and rebar, jumped to several other things inside the house, including AC wiring which it blew out in a few places, and it found a metal corner protector on a wall, where it traveled down to a tile floor and blew a hole in the floor, blowing tile pieces thru two rooms. 

The damage was extensive including all the station equipment (which was not connected to the antenna).  There was damage to almost all electronics in the house and the central air/heat. The house slab and the plumbing could not be examined for damage.  Insurance companies in Florida, because of all the large claims they have had to pay recently, look for reasons to cancel your insurance.

There are ways to survive lightning strikes without damage, but you have to have a path for the lightning that can carry the current without disintegrating. You also have to do all the grounding and ground wiring correctly.  My tower gets hit 2 to 3 times per year average with no damage, so far.  It's still possible to sustain damage if the lightning ignores the big target I give it, and hits something else.  That's much lower probability, but not zero probability.  I have seen lightning violate the "cone of protection rule", but that doesn't happen very often.

There are areas of the country where lightning density is low, and local terrain is not conducive to lightning strikes, and if the antenna is low and surrounded by higher objects, then the risk of having an unprotected system may be justified.  Whatever the situation, it is the ham's responsibility to decide to take that risk, or not.   First step is understanding the risk.

Jerry, K4SAV
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