Old Ben Franklyn really knew something. :)
73 -- Larry -- W1DYJ
-----Original Message-----
From: Patrick Greenlee
Sent: Thursday, January 29, 2015 8:02
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Antenna to Shack Ground Connection
Your story seems to support the old rule of thumb. Lightning hit all
around but not directly on to the tower.
Lots of folks think lightning rods (and other structures such a towers
etc.) are intended to attract lightning to protect other objects from
being hit. Not so, gentle reader, as the primary purpose of lightning
rods is to drain away static charges such as accumulate and thereby
prevent a lightning strike. If the grounded system is hit by lightning
then the protection system has failed its primary goal. A secondary
consideration is surviving the strike so as to continue with its primary
purpose of draining charge and preventing strikes in the immediate area.
The videos we recently watched showing guys climbing tall towers let us
see some of the charge draining structures. Some looked like metal
modern art impressionistic porcupines among other styles. The shared
attribute of the charge draining structures was their radius of
curvature. Sharp points and edges to allow charges to more easily drain
away. The electric field vector strength is inversely proportioned to
the radius of curvature of the conductor. Sharp points "amplify" the
field strength leading to more ionization and better conduction to drain
the static charge.
Patrick NJ5G
On 1/28/2015 10:13 PM, Roger (K8RI) on TT wrote:
On 1/28/2015 1:57 PM, Patrick Greenlee wrote:
Not a debunking, but maybe a modification<:-))
I have a 100' 45G that had a 30' mast extension in the center of a
200 X 200 lot. One multiple strike abt 6, or 7 years ago hit all
around us except to the S, yet it ignored my system that was almost in
the center of the strike zone.
http://www.rogerhalstead.com/ham_files/Tower29.htm I was in a bit
better shape back then. Yet as I've mentioned a number of times, it
took 17 visually verified hits in 6 years and nothing in the 8 years
since. Oh! That strike did a lot of damage in the neighborhood.
The only thing predictable about lightening, is its unpredictability.
73
Roger (K8RI)
The old rule of thumb for lightning protection from over head
dissipation devices (high metal roofs, lightning rods, grounded
towers, etc. is a 30 degree cone angle. This is NOT a guarantee but
a tendency. I think more is better and would not find fault with a
ground wire a ways above coax or control leads but would encourage
ground rods along the way so as to not encourage a great deal of
coupling between lightning generated ground wire and "other"
conductors below it.
Patrick NJ5G (now stand by for a total debunking of my comments...)
On 1/28/2015 12:30 PM, David Gilbert wrote:
I don't have the expertise to be sure about it, but it always seemed
to me that it would be best to run the ground wire (grounded at both
ends and maybe a few places in between) in the same trench as the
coax ... but some distance above them ... in order to provide a sort
of shielding effect for the coax against induced currents from
nearby lighting bursts. I was once told that the electric utilities
run a ground line at the top of utility poles above the actual power
lines for a similar purpose.
Possibly induced currents in the ground wire would still couple to
the coax anyway, but I certainly don't see how running the ground
wire with the coax would be any worse than running them separately.
73,
Dave AB7E
On 1/28/2015 8:43 AM, Cox, Norman R. wrote:
Dear Group:
I am now ready to connect the tower grounding rod network to
the shack grounding system. The crank-up tower is about 30 feet
from the shack. Is it preferable to run the ground connection
between the two along with signal cables? Or is it better to run
the grounding connection not so close to the signal cables? I've
seen people do it both ways, and can see a possible benefit either
way, but a lightning strike could throw either idea out the window.
Thanks,
Norm
KE0ZT
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