Honestly Rick, I dissagree (nicely).
It just does not make sense to me otherwise. Now I have only had about 2 years of physics in college, so I am not the expert in any of this... far from it. But I cannot see how having it at a level lower than another close conductive source in the same proximity will benefit it. In fact, I think it will be a detriment.
Since the bottle brush basic method of operation is to dissipate the charge which will accumulate on the easiest path to ground (ie, typically the highest point), having it below the easiest path to ground would not benefit it. Though on a large strike it may still have some benefit as the entire top of the mast will be gettting charged, it would be better suited by placing it higher than any corresponding conducturs/charge accumulators. I can see, as they wrote up in their web site, that you could place it too high... ie, ten feet or higher where there is a conductive loss between the easiest point to ground and the dissipator - which would decrease the dissipators effectiveness. However, on a sailboat where everything is in very close proximity, that does not make sense.
As far as the 6 inches, I meant for a lightning rod. My typo and mistake. It would not hurt for a bottlebrush too, but that was in the instructions I had for my lightning rod.
In general, using a faraday cage, I would see the top of the
VHF antenae as the top of your cone. Given the buildup of the charge on top of your mast and the fact that the dissipator is lower, I would see the top of your
VHF antenae as the top of your cage and the point where the strike would occur. If I were you, I would lower the antennae and add a lightning rod instead. THere seems to be some debate whether it shoud be very sharp or blunt... who knows?? But that is what I would do. I looked at the Forspar and other sites and they also reccomend the disspator being the highest point. Sailnet also sails it and says the same thing. Lightning Master is the only site that says it does not HAVE to be at the top - but there statement is based upon observation and not scientific, proven experiments. I also think (THINK) that they are being a little sarcastic when they say it may have a detrimental effect, "... when mounted too high..." as in a height well above what would be practical in a marine application... especially a sailboat.
The whole science behind disspiating a charge is pretty debated. I think a disspator of one type or another is a benefit, but I think the prudent sailor should be even more concerned with finding a clear and direct path to ground where the boat and its occupants would not be injured. A dissipator, in theory, only lowers the chance of a strike. It does not remove it.
Just my opinions, Rick.
- CD
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