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Re: [TowerTalk] Arrestor placement

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Arrestor placement
From: "Roger (K8RI) on TT" <K8RI-on-TowerTalk@tm.net>
Date: Sun, 28 Dec 2014 23:05:20 -0500
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>


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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