[TowerTalk] grounding compromises
RedHaines at centurytel.net
Sun Aug 6 23:39:41 EDT 2006
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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