[TowerTalk] grounding compromises
RadioIR at charter.net
Mon Aug 7 01:23:17 EDT 2006
..."If the Feedline is #12 or larger copper, it will carry
the current of 90% of strikes and will fail open in the rest."...
If you believe this, you haven't seen many lightning strikes. If a #12
wire is the only path for a stroke, it will simply disappear. You will
not be able to find even the smallest trace of copper left anywhere.
You may find some of the insulation. It will be blown apart so rapidly
that it doesn't even have time to melt.
You should see what it does to ladder line. Insulation rains down like
small bits of confetti, with no melt marks.
You can't control current flow when you have no path for it to go.
>The dipole can be protected as effectively as other ungrounded
>antennas. Of course, the antenna, being at a higher elevation than
>other station components and utility lines, is certainly a target of any
>lightning strike in the vicinity, and is an effective receiver of energy
>from strikes that don't hit it directly.
>Fir information about the distribution of number of strokes, current in
>a stroke, and other lightning statistics, refer to _Lightning: Physics
>and Effects_, Dr. Vladimir A. Rakov and Dr. Martin A. Uman. 2003.
>Cambridge University Press. This is an undergraduate text, not too
>technical , that provides useful explanations of why lightning does what
>it does as well as the latest findings and data from lightning research
>and information regarding lightning protection. It is not commonly
>available at local public libraries. I borrowed a copy via
>inter-library loan from U of MN.
>For information about protecting equipment from lightning, see:
>_Protection of Electronic Circuits from Overvoltages_, by Ronald B.
>Standler, 1989; John Wiley & Sons, Inc., particularly Part 2, Chapters 7
>through 15, for information about gas tubes, varistors, avalanche and
>Zener Diodes, semiconducor diodes and rectifiers, thyristors, impedances
>and current limiters, filters, isolation devices, and parasitic
>inductance and how these devices may be used to protect electronic
>equipment from overvoltage stresses such as those associated with
>lightning. This book may not be available in many public libraries but
>is available on inter-library loan from college and university libraries.
>When the lightning surge reaches the lightning arrestor, it is partly
>bypassed to ground by the arrestor and partly reflected back to the
>antenna by the mismatch caused by the arrestor as it reacts to the surge.
>If a small guage feedline is used, it may fail open, but the arrestor
>will do its job. If the Feedline is #12 or larger copper, it will carry
>the current of 90% of strikes and will fail open in the rest. There may
>be sustained arcing at the point of failure, but that will help
>dissipate the energy. Just make sure the lightning arrestor is applied
>before the feedline gets into the house, so the failure of the wire will
>be outside the house.
>Unless an arrestor of some form is placed at the point where the
>feedline attaches to the dipole, the insulation of the feedline may be
>damaged. A gas tube between the dipole elements is adequate at that
>point. A three terminal gas tube that shunts the dipole elements to
>each other and to ground is probably better. Incidentally, the three
>terminal gas tube shunts all three terminals together, with 15-25 volt
>differences, simultaneously when it is triggered, whereas individual two
>terminal gas tubes are unlikely to trigger simultaneously. See Standler.
>It is appropriate to use one of the lightning arrestors that includes a
>blocking capacitor, like PolyPhaser or ICE at the entrance panel if coax
>is used for feedline.
>If open wire or balanced feedline is used, a special lightning arrestor
>that uses a three terminal gas tube between feedline wires and ground
>and a blocking capacitor in each feedline wire going to the station is
>required. I haven't confirmed that either of the vendors mentioned uses
>such a gas tube, but they are available, as discussed in Standler.
>Lightning is mysterious in some ways, especially in how it is created
>and what triggers a strike, but its characteristics, once a strike is
>triggered, have been measured and studied. Those characteristics are in
>accordance with the known laws of physics and the information is
>available in a number of publications.
>73 de WOØW
>>Most people on this reflector are used to thinking in terms of grounding
>>towers, not dipoles. The same rules apply to dipoles, but there is
>>another problem with dipoles that puts it into the hard pile.
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