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post #21 of 45 Old 03-10-2016
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Re: VHF Antenna Placement

Of course, if you're worried about your electronics, you can stow them in the oven, or even try an ESD bag.
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Re: VHF Antenna Placement

chris-
The only sure protection from lightning strikes is building a huge faraday cage, or moving deep into a cave.
In theory you can put up a lightning rod on the masthead, either "next to" the VHF but separated (i.e. using a masthead fitting a foot or two wide) and sufficiently taller. Most boats wouldn't attempt them. So in practice, putting the antenna below the masthead, on the side of it, is common. A lot depends on your rigging and where you can put it without tearing sails on it. This is a compromise.
More importantly, if you install lightning protection like a PolyPhaser on the coax, your antenna and coax may be toast but the rest of your equipment should survive. Another option is to have a physical disconnect in the coax as it comes below deck, and to keep it plugged into a direct ground cable when the VHF is not in use, i.e the boat unmanned. That will ensure any strikes go to ground, and not your radio and electronics. Cheap and simple.
You might also consider getting one of the inexpensive "emergency" antennas to keep as a spare. If your only damage is to your antenna aloft, and the cable, then one of these will get you back on the air well enough until you can make proper repairs from the strike. Again, cheap enough.
Also try to use good cable (another whole debate, there are threads on that) and make sure it is installed and supported properly. And weatherproofed at all connectors. The antenna is useless, without a good cable.
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post #23 of 45 Old 03-10-2016
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Re: VHF Antenna Placement

or better yet divert the lighnening to the flux capacitor and use it for time travel


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post #24 of 45 Old 03-10-2016 Thread Starter
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Re: VHF Antenna Placement

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Originally Posted by SVAuspicious View Post
Okay start with a simple model. Tens of thousands of volts (pick a number, say 50,000 V) and hundreds of thousands of amps (pick a number, say 500,000 A). Pick a really big wire for your new aluminum mast, say 6 AWG, and look up the resistance per foot. Look at the cross-sectional area of the mast and estimate the resistance per foot. A little E=IR and a little P=IE. Pick an ambient temperature and look at the temperature rise in the material and compare to the melting point. Include the cross sectional area of all your rigging. Take credit for high humidity atmosphere (the model is getting more complicated) and even rain on the mast and deck. Look up the melting point (and sublimation point) of epoxy resin.

What you are going to find is that the path is indeterminate. We can play with macroscopic Maxwell's equations but we are seriously into non-linear behaviors over milliseconds.

It boils down to being hit by lightening is very very bad.

For empirical data, take a look at post event pictures of wooden masts hit by lightning. Look at masonry chimneys hit by lightning. Trees.
In the absence of any numbers this isn't a particularly convincing argument. But I get it - your opinion is that its pointless to do anything.

Listen, you may be right!

I think its a little less hopeless than you are making it out to be though.

By the way, 0AWG copper (little over a quarter inch diameter I think), a commonly specified lightning down conductor, has a resistance of about 0.0001 ohms per foot of wire. Assume you've wired that up to the top of your mast, so its a, say, 50 foot long down-conductor. Next, google will tell you that the average bolt of lightning carries 30,000 amps of current. V = IR, so the voltage drop across your down conductor when it's carrying that load is 30000 * (50 * 0.0001) = 150 volts!

Thanks to your trusty down conductor, you've pretty dramatically neutralized the problem, at least in the space between the top of your down conductor and the ground.

I don't know how to calculate how long that wire can handle that current before it gets too hot to be useful, but a lightning strike is not a long event (10s of microseconds).

OK so most of us are just using our mast and our rigging as our down-conductors to get to deck level, not a big fat 0AWG copper cable. Luckily the tradeoff in conductivity per cross sectional area is made up for by, well, a lot more cross sectional area! In my case, the total area of my uppers (made of stainless) and the mast section (made of aluminum) *blow away* the current carrying capability of a 0AWG copper cable.

And as a final point, lightning protection systems are almost ubiquitous on tall structures in lightning prone areas, and there are guidelines for sizing lightning down-conductors, and you simply aren't going to convince me that its all malarky. That's all I meant by "I dont want to debate this".

All that and it may still not work, but its certainly not as futile an endeavor as you make it out to be.

After all that, sticking an antenna up on the top of the mast and wiring it straight into your living quarters certainly feels silly. Which, of course, is the point I would /like/ to discuss with anyone else who has considered it.
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post #25 of 45 Old 03-10-2016 Thread Starter
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Re: VHF Antenna Placement

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Originally Posted by hellosailor View Post
chris-
The only sure protection from lightning strikes is building a huge faraday cage, or moving deep into a cave.
In theory you can put up a lightning rod on the masthead, either "next to" the VHF but separated (i.e. using a masthead fitting a foot or two wide) and sufficiently taller. Most boats wouldn't attempt them. So in practice, putting the antenna below the masthead, on the side of it, is common. A lot depends on your rigging and where you can put it without tearing sails on it. This is a compromise.
More importantly, if you install lightning protection like a PolyPhaser on the coax, your antenna and coax may be toast but the rest of your equipment should survive. Another option is to have a physical disconnect in the coax as it comes below deck, and to keep it plugged into a direct ground cable when the VHF is not in use, i.e the boat unmanned. That will ensure any strikes go to ground, and not your radio and electronics. Cheap and simple.
You might also consider getting one of the inexpensive "emergency" antennas to keep as a spare. If your only damage is to your antenna aloft, and the cable, then one of these will get you back on the air well enough until you can make proper repairs from the strike. Again, cheap enough.
Also try to use good cable (another whole debate, there are threads on that) and make sure it is installed and supported properly. And weatherproofed at all connectors. The antenna is useless, without a good cable.
Hey - these are awesome options, thanks! This is exactly the kind of tip I was hoping for.
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post #26 of 45 Old 03-10-2016
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Re: VHF Antenna Placement

I still think that over-engeenering is not the way to go. Building a seperate lightnkng protection involves a system that adds complexity to the mast rig. It's not practical. I do not sail (often) in areas of lightning, so my experience is limited. However, I have read about the traditional manner of dealing with this:


The traditional way to deal with it is to have a large metal cable attached to a shroud and trailing into the water. You put this cable into play when the conditions call for it. The mast is already the biggest lightning rod up there. The shrouds are the biggest wire. Allowing the path to the water on the outside of the boat creates the path of least resistance to be outside the hull. Do any of you have experience or comments on this method? I also agree that carrying a spare vhf, cable and antanee would be prudent. I think it would also be cheaper, easier and more effective than building a complex system. It also works for your "spare" GPS! What do you think? Put them in the oven for a "Fariday" cage?

Last edited by Scotty C-M; 03-10-2016 at 05:02 PM.
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post #27 of 45 Old 03-10-2016
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Re: VHF Antenna Placement

The only reason to think that moving your VHF Antenna will reduce the damage to your boat, is that for some reason you would reduce the probability of taking a strike to your boat.

Given that you have a tall (with respect to water) conductor that has sharp edges you will have ionization at the masthead independent of the antenna pointing skyward, the question is whether you would have charge build up sufficient to attract a strike.

You could put the antenna on your mizzen, and put a lightning dissipator (Lightning Master Static Dissipator) on your main, and hope it would make a difference, in bleeding off charge. Since the VHF Antenna acts as a pretty good ionization point, you could argue it's doing the work of the dissipator.

If you are worried, take a pair of jumper cables and clip to your upper shrouds and toss the ends in the water, and get inboard away from stays and shrouds.
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Re: VHF Antenna Placement

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Originally Posted by DRFerron View Post
I think I can settle this. A few years ago I took a yacht insurance seminar. He said that catamarans submit more claims for lightning strikes than monohulls.

Seems clear that the answer is not to buy a catamaran.
Thanks for this info i will never anchor next to a Cat in a thunderstorm again
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post #29 of 45 Old 03-10-2016 Thread Starter
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Re: VHF Antenna Placement

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Originally Posted by nhsail View Post
The only reason to think that moving your VHF Antenna will reduce the damage to your boat, is that for some reason you would reduce the probability of taking a strike to your boat.

...

If you are worried, take a pair of jumper cables and clip to your upper shrouds and toss the ends in the water, and get inboard away from stays and shrouds.
After moving inboard, you look to your left and see your VHF cable a foot or two away, wired straight up to the top of your mast. Which, of course, is the whole point of this thread. ;-)

Last edited by chris95040; 03-10-2016 at 05:32 PM.
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post #30 of 45 Old 03-10-2016
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Re: VHF Antenna Placement

The best thing to do is keep a good eye on the weather and pray a lot. I was about 100' from a fishing boat that got struck all aluminum boat and it fried all the electronics but no one on board got hurt, but there was a line at the head after the strike. I try to grab the Admiral and do something to take our minds off the storm and pray some more. I do have a lead keel and all shrouds and stays and the mast are grounded but in a 25' boat there is not many places to hide.
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