Also see:
Contesting and Boatanchor Room
Antenna System and my house station
Damage-prone installations almost always
include one or more of the following mistakes:
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The following are popular false myths....
Examination of rumors and old wives' tales, or Internet wisdom, shows us for nearly every claim there is a countering claim. What do we do, isolate everything? Ground everything? Use no ground at all? Use a rod? Let charges bleed off? Stop charges from bleeding off by not grounding? To answer this we have to understand where charges are, and how to deal with discharges.
Our worry isn't so much about charges piling up, it is about large charge
differences between two points. A charge difference creates a charge gradient or
electrical potential between two or more points. Eliminating the potential
difference requires we move charges from one point to another, balancing charges
out.
The charge
difference is
between different
areas of clouds, and
between those clouds
and the entire earth
and anything near or
on the earth. The
real problem is the
piling up of
electrical charges
in the droplets (or
even dust particles
in bad dust storms)
collected in one
area or another of
"clouds".
If we wanted to
reduce the charge
gradient, we would
have to create a
conductive path
capable of allowing
steady charge
movement
between the areas
with different
charge, so charge
difference could
equalize. It is
quite possible to
reduce charge
gradient between an
airplane and the
surrounding
atmosphere by
attaching wire
brushes or whiskers
to protruding parts
of the airplane.
As the perfectly-insulated airplane
travels directly
through differently charged
areas, charges can
easily migrate into
or out of static dissipators, bringing
the airplane to the
same potential as
the area it is
flying through. This
is much the same as
a conductive strap
on a well-insulated
motor vehicle
contacting the
surface of a road,
preventing tire
friction from
charging the
insulated vehicle
to a different
charge potential in
relationship to the
road (and earth).
It's my firm belief,
based on both
reasonable logic as
well as several
reports and studies,
that grounding an antenna
(or adding metallic
whiskers or
metallic "porcupine balls")
does nothing to reduce charge
gradient that causes
lightning. Moving
charges between the
huge volume of the
earth through a
tower into the air
immediately
surrounding the
tower and earth is
meaningless, because
the real charge
gradient is between
clouds thousands of
feet away and the
entire earth.
Unless the discharge
contacts or forms a
path connecting differently charged
areas, the ionized
area does
nothing at all. The
most it offers is a
wider blunter area
at the location of
the dissipater.
While this might
slightly increase
the the voltage
gradient of the area
immediately around
the dissipater to
the sky, it does not
alter the charge
gradient or voltage
between the cloud
and tower.
Everything I have read detailing successful deployment of lightning charge dissipators has been anecdotal at best. In every case there are several alternative explanations that have been ignored. For example, several people have told me beacon or lightning system damage has been reduced by addition of a mast and dissipators around a beacon light. On the other hand investigative reports that collect data, including data from NASA, find lightning strikes occur at similar rates with or without dissipators.
In my own systems, I have a commercial metal antennas or metal masts mounted above my beacon lights. Looking at my Rohn 65G to the left, you can see almost 15-feet of antenna above the beacon light. The "thin" black line going horizontally to the right is a 160-meter dipole mounted at about 310 feet. The more vertical lines are ropes, and the fiberglass upper guy lines are clearly visible.
Prior to the installation of the upper VHF antenna, with only a commonly-used short spike sticking a few feet above the beacon light, I replaced several MOV's and tower flashing modules in the tower lights.
After installing the antenna protruding 15 feet above the beacon light, and bonded to the tower below the beacon light, this tower has taken hundreds of strikes without damage to any electronics.
The taller low-impedance conductor bonded to the tower below and away from beacon wiring reduces damage to things lower on the tower. Lightning current is harmlessly routed around the beacon, rather than flowing through a thin lightning rod mounted immediately next to the beacon and grounded to the beacon mounting plate. This antenna produces the very same improvement credited to fancy expensive whiskers, without the need for false tales about "charge dissipation" or "lightning mitigation" or "charge equalization".
A second effect of adding the mast above the beacon was reduction in damage to my 160-meter dipole. Without the tall antenna mast at the top, the coax in the 160 dipole's balun would occasionally melt during a strike. After installing the tall mast, there have been no balun failures.
Had I installed lightning dissipators, I would have probably credited these improvements to mitigation or reduction of strikes. Unless I was watching the tower throughout a storm, the only way I would know if the tower was hit would be by observing damage to equipment on the tower after the storm passed.
The problem we face is the small cloud mass far away from our massive earth is charged more and more as a storm progresses. The cloud either has to stop charging before it reaches a voltage breakdown point, or there must be a direct path that allows it to equalize charges without doing damage. Nature eventually takes care of this. When the charge gradient between the cloud (the source of the potential) and the earth (just a big charge sink or reservoir) becomes large enough, a streamer forms and paves the path for full blown lightning bolt.
Connecting an
antenna to earth
does nothing at all
to reduce the
likelihood of a
strike. The antenna
is already at earth
potential, the real
problem is the huge
potential difference
between the cloud
and earth. The tower
is simply a
protrusion that
lowers the breakdown
voltage between the
cloud and earth. As a
matter of fact,
grounding if
anything only makes
the problem ever so
insignificantly worse. A
grounded antenna is
solidly clamped at
earth potential,
instead of being
ever so slightly
closer to cloud
potential like an
insulated or
electrically
isolated antenna
could be. In the
large scheme of
things, none of this
affects the
likelihood of a
strike. What big
improvement would
come from several
thousand volts of
change when compared
to millions of volts
of potential
difference? The only
significant change,
if we want to reduce
direct hits, is by
reducing structure
height.
A second (but less
effective) way to
reduce how
frequently a target
is stuck is to
create a very
wide blunt target.
Having a blunt
target will
reduce the electric
field density
appearing
at one concentrated
point. This is the
same effect that
causes a wider gap
in a spark plug, or a
blunt smooth tip in
a spark plug, to
greatly
increase gap voltage
breakdown. Grounding
the shell of the
spark plug better does
not help increase
gap voltage breakdown,
and neither does
putting sharp
whiskers on the
electrode tip!
Other than reducing
tower height until
it is well below the
height of
surrounding objects,
there really is only
one reliable course
we can take to
reduce damage risks. We can provide
a low impedance path
to a wide area of
earth, routing
lightning
current around things that can be
easily damaged.
Installing towers a
reasonable distance
from buildings is a
good idea, as are
perimeter grounds
and proper cable
entrances. I've seen
some terrible
structural damage,
and even one fire,
caused by
terminating guylines
or tower support
brackets into
building walls. Steel guylines
should be terminated
at earth anchors
that are reasonably
well away from
buildings or
building walls. If
that isn't possible,
use fiberglass
guylines or install
low-impedance
earthing systems
that are tied into a
building perimeter
ground. Bracketed
towers need good
grounds at the tower
base, and house
brackets should not
be near large
metallic objects in
the house. The last
thing we want is a
tower bracket arcing
through a dry wooden
attic surface to a
metal duct or attic
electrical wiring.
Anyone who thinks a
few six-foot or
60-foot deep ground
rods can dissipate
hundreds or
thousands of amperes
at frequencies from
near dc up to radio
frequencies with
negligible impedance
probably should
spend time rethinking
the frequency
spectrum of
lightning. Unless
there is a very
large highly
conductive surface
area, like a radial
field or ground mat,
it
is almost impossible
to even spread or
dissipate strike
current. With small
area grounds, no
matter how deep they
are or how good they
are at dc or 60 Hz,
there will be a
huge voltage rise
between different
points around the
strike. We have to
do our best to have
everything
in the protected
area rise in voltage at the same
same rate. Keeping
charge levels of
different things
rising and falling
at the same rate is
the primary reason we
must bond the
utility entrance
ground to the radio
shack entrance
ground. It is the
reason a
protected area
requires a low
impedance perimeter
ground buss encircling
the entire protected area.
We can't make problems go away or reduce the odds of a strike noticeable amounts by grounding or snake oil cures like static dissipators. We can't discharge the clouds intentionally. We just have to deal with what happens during a strike.