[TowerTalk] Arrestor placement

[Roger (K8RI) on TT]

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 at arrl.net
> web: http://wiki.k1ttt.net
> AR-Cluster node: 145.69MHz or telnet://k1ttt.net
>
>
> -----Original Message-----
> From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of David Gallatin via TowerTalk
> Sent: Saturday, December 27, 2014 18:49
> To: TowerTalk at 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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