On 8/1/2010 4:02 PM, Tommy Alderman wrote:
> This then, means that ICE lightning protectors, with their center conductor
> at DC ground, are really not good protectors to use? Doesn't it also bring
> in to question the advice to connect coax shields to both the top and bottom
> of a tower?
>
> To me, what Dave says, answers the age old question about antennas of any
> kind: 'To ground or not to ground?".
>
>
When charged clouds pass overhead, there is an equal and opposite
"charged area" (more or less) following along under the cloud on the
ground. That includes all objects such as homes, cars, people, and
towers whether they are grounded or not.
What you do to the tower has very little effect on whether it's going to
get struck or or not. What you do *does* affect what happens when the
tower does get struck and the results of that strike in the ham shack or
house. Grounding the coax to the top and bottom of the tower keeps the
coax at what ever potential the tower is at those points. Antennas that
keep the center conductor at DC ground potential may or may not help,
BUT it will certainly help with the many thousands of volts generated by
precipitation static. Also that center conductor possesses inductance
and capacitance to the shield. So on longer runs the two will tend to
move to the same potential. That is why we put the Polyphaser of Ice
protector at the grounded bulkhead. Coax that is not grounded to the
tower at the top and bottom can actually have more voltage induced into
it while at the same time the tower is dumping the current into the
ground system. I have seen "snow static" create a big fat arc that
actually extended out away from the PL-259 before reaching the shield
and sounding like a balloon popping every 5 to 10 seconds. So instead of
a half inch spark it was an inch or two long and far fatter than any
spark I ever saw from an ignition coil. Had that 40 meter vertical been
grounded it would not have had that build up.
This is one of the reasons when you disconnect the coax from the rigs
before a thunderstorm, you need to get the end of that coax out of the
house/ham shack. Ideally the end of the coax should be terminated on a
grounded connector before it enters the shack. If that antenna is
struck by lightning, or even has an induced voltage from a nearby
strike, that cable laying on the floor may have hundreds of thousands of
volts on it. That voltage is likely to jump to the nearest object or
lesser potential, or grounded objects. That could be a person or the
rig and it could create an electrical shock or fire. Hence the reason
for the grounding bulkhead where the coax enters the house. If the coax
is not grounded where it enters the house, throw it out. You are
repeating Franklin's experiment with much better conductors than wet
string. Of course by what we know now, Franklin was extremely lucky not
to have been killed.]
For those who just run the coax through an ungrounded bulkhead under the
window, I know the thought of throwing the coax out into the elements
where the end is likely to get wet and dirt in it is a terrible thought,
but not doing so is like playing "Russian Roulette" with your house and
body at risk.
I've said it on here many times, but the first 5 years I had the tower
up it was struck by lightning an average of 3 verified times per summer
or a total of 17 times the first 6 years. It has not been stuck at all
(that I know of) over the past two summers nor so far this summer.
One additional point is that a good and adequate ground system enables
the tower to sink all (or most)of that current and voltage from a
strike, into the ground instead of having it run down the coax and into
the house.
73
Roger (K8RI)
> Tom - W4BQF
>
>
> Subject: Re: [TowerTalk] Vertical antennas and lightning
>
> Unfortunately in this case the 'bleeding' when there is a charged cloud over
> head is putting charge ONTO the antenna from the ground to equalize it with
> the rest of the charge being attracted to the base of the cloud. So it make
> it MORE likely to start the upward streamer that completes the path.
>
>
> David Robbins K1TTT
> e-mail: mailto:k1ttt@arrl.net
> web: http://www.k1ttt.net
> AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
>
>
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