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#1
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Lightning protection question revisited
I am considering connecting a coil from the base of my vertical antenna to
ground, not for impedance matching, but to allow the charge on the ground to slowly leak off at the top of the antenna. I will also use a lightning arrestor but wonder if this additional step will further reduce the chance of a lightning strike. About a week ago I posted the above message. Several people responded and there was a general feeling that the idea of allowing a gradual leak off of charge at the top of the antenna would not prevent or even be likely to reduce the chance of a lightning strike. Tonight I was looking through my old "Elements of Physics" by Shortly and Williams, fourth edition, page 697 and came across the following statement: "The action of a well-grounded and sharply pointed lightning rod is primarily to discharge quietly into the air the charge that is induced in the surrounding earth by a charged thundercloud above. The electric intensity in the vicinity of the sharp point is high enough to break down the immediately surrounding air and the continual discharge prevents building up sufficient voltage over the whole cloud-to-ground path to break down this long path and give rise to a stroke of lightning at this location." Comments? Ron, W4TQT |
#2
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Ron:
Because lightning is a difficult process to study, you will find that the references change with time, as more is learned. Blue jets and red sprites come to mind, for example. One startling example is the National Fire Protection Agency Lightning Protection Code. The one I used and became familiar with in the 80s and early 90s was withdrawn and canceled when it was understood that some of the premises on which it was based were simply wrong. The new code, which has finally been released, is very different from the previous one. I no longer have current references to this discipline available, but I suggest reading the recent issues of Journal of Atmospheric and Terrestrial Physics, and Journal of Geophysical Research for more up to date information than an old textbook. The fact is that the phenomenon you cite does not bleed off anywhere near enough charge to make any difference. Yes, it does drain off charge. No, the quantity is like trying to empty the ocean with a leaky coffee can. -- Crazy George Remove NO and SPAM from return address "Ron" wrote in message ... I am considering connecting a coil from the base of my vertical antenna to ground, not for impedance matching, but to allow the charge on the ground to slowly leak off at the top of the antenna. I will also use a lightning arrestor but wonder if this additional step will further reduce the chance of a lightning strike. About a week ago I posted the above message. Several people responded and there was a general feeling that the idea of allowing a gradual leak off of charge at the top of the antenna would not prevent or even be likely to reduce the chance of a lightning strike. Tonight I was looking through my old "Elements of Physics" by Shortly and Williams, fourth edition, page 697 and came across the following statement: "The action of a well-grounded and sharply pointed lightning rod is primar ily to discharge quietly into the air the charge that is induced in the surrounding earth by a charged thundercloud above. The electric intensity in the vicinity of the sharp point is high enough to break down the immediately surrounding air and the continual discharge prevents building up sufficient voltage over the whole cloud-to-ground path to break down this long path and give rise to a stroke of lightning at this location." Comments? Ron, W4TQT |
#3
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"Crazy George" wrote in message ...
Ron: The fact is that the phenomenon you cite does not bleed off anywhere near enough charge to make any difference. Yes, it does drain off charge. No, the quantity is like trying to empty the ocean with a leaky coffee can. Very true. Also, you sure wouldn't want to rely on that discharge theory to prevent a strike. Will be the other way around if it's a pointed tip. It will be struck sooner or later. But, if that tip were hit by lightning, the lightning would have struck in that close area anyway. I have tall trees in the yard in both the front and back. The last two close strikes, which BTW, have been in the last few weeks, have hit the trees instead of my mast. But my mast is not pointed. It's just pipe. Not as good a streamer as a point. Lightning has no idea where it's going until it gets to about the last step of it's travel and finds a streamer to hook up with. In a strike zone, there will be many, many streamers poking up all looking for attention. Which gets the call is akin to throwing a dice. Many times, the best streamer in the area is not in the right location to get the job. I always assume any object I stick up in the air will be struck sooner or later. To think anything else is foolish. The brush dischargers work on the same "theory". They are still struck at times. Trying to discharge fast enough in a storm to avoid a strike is like taking a whiz in a whirlwind. You may still get something on you...:/ MK |
#4
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Ron, W4TQT wrote:
"---Shortly and Williams, fourth edition." Hang on to your Shortly and Williams. That was one if my textbooks long ago and I found it usually reliable. There are lightning elimination companies which erect spikes said to drain the charge difference at your site. This, they say, drains your site of charge which might initiate a strike. Yet, I`ve seen towertop lightning damage where the tower was the most salient item in the area. So, why didn`t it successfully discharge the site and obviate the strokes? Cecil`s 400-pound gorilla analogy may apply. Protection relies upon residence within a conductive enclosure and clamping the voltage level on every conductor which penetrates. Best regards, Richard Harrison, KB5WZI |
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