I agree with David, but I've not read all of the thread.
You want the arrestor as close to the thing it's protecting as
practical. The common point ground at the station entrance is the best
compromise. I'd phrase it a bit differently than David, but the result
is the same. The effects of a spike coming in as induced voltage in
the house wiring depend on a number of things. An induced spike will
likely create a substantial spike on the power, phone, cable, and the
antennas. You have the direction to the source, or lightening bolt. The
induced voltages will travel a bit slower on the wiring than the EMP
through the air. So distance and direction will determine the phase
differences between all these conductors connected to the rig.
The pulse "induced" into the antenna(S) from a nearby strike sees the
coax and tower as parallel conductors while the center conductor of the
coax acts like one plate of a very long capacitor so the voltage across
the coax is much less at the base of the tower than at the top. With
the coax braid grounded to the tower at the top and botton, both are
"Earthed" at the base of the tower. With the coax between the tower and
shack the coax agan acts like a long capacitor reducing the center
conductor to shield even farther. This in itself reduces the work the
arrestor has to do substantially. (Another reason for mounting the
arrestor at the CPG.
Beyond that being the best place to put the arrestor, lightening
protection can never guarantee 100% protection. The induced voltages
come at the rig from all directions with their phase differences and
amplitudes varying from almost nothing to those we don't want to think
about.
Lightening protection is a crap shoot, BUT, a properly placed lightening
arrestor along with a well laid out ground system of sufficient size
pushes the odds in our favor. It can be substantially in our favor,
however you soon reach the point of diminishing returns where where it
takes a LOT invested to only give a little increase in protection.
I had no damage from many direct strikes to the tower (17 visually
verified strikes and who knows how many were not seen) along with major
close strikes that did costly damage to the neighbors. The one was a
multiple strike that hit all around me, but ignored the tower and
rigs...BUT...Last summer a nearby strike got into my CAT 5 gigabit
network and did major damage to several computers. I ended up
rebuilding two BIG computers, while the router and switch were fried..
One was a complete rebuild from the motherboard up. The other, a smaller
computer, but still larger than what most would have was a complete
replacement. The one computer hooks into the station, but I've found no
damage to the station.
Unfortunately I use OEM software which meant not only new hardware, but
new operating systems as well. The good news was the station had no
problems, or at least none I've found so far.
73
Roger (K8RI)
On 12/27/2014 1:58 PM, David Robbins wrote:
Yes, it can make a big difference. Lightning arresters are NOT just protecting
you from a strike on your antenna. They also protect you from a strike on the
power line or even just to the ground nearby that causes the ground system
voltage to rise while the voltage on the center of the coax doesn't... if the
arrester is not very close to the radio it delays the arrester action which can
make your radio the easiest path from ground to the center conductor... in hv
engineering terms its called a backflashover, which is caused when lightning
hits the ground system causing a flashover to a power conductor.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of David
Gallatin via TowerTalk
Sent: Saturday, December 27, 2014 18:49
To: TowerTalk@contesting.com
Subject: [TowerTalk] Arrestor placement
Hello all,Somehow I have gotten the idea that one's lightning arrestors are
best placed at the bottom of the tower. This seems logical to me. Then I came
across another piece that said, no, arrestors are to be placed at the shack
entrance panel. Which is it? Or does it matter? 73, David, AA9G
ex W5DCG and KC9EEV
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73
Roger (K8RI)
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