> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Jim Brown
> Sent: Sunday, September 12, 2010 12:23 AM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Lightning Protection Question
>
> On Sat, 11 Sep 2010 21:28:55 -0400, Gene Smar wrote:
>
> >You overlooked my caveat in my original e-mail: I have a remote
> >> switch
> >> in the NEMA box at the base of my tower. Had I installed the
> Polyphasers
> >> at
> >> the shack end, only one coax run (between the shack and switch) would
> have
> >> been protected; the switch and antennaa coax runs into it would have
> been
> >> vulnerable to lightning damage. I installed the Polyphasers in the box
> on
> >> the tower so as to provide protection for the switch and all three coax
> >> lines going into the switch.
>
> Gene,
>
> I didn't misread or overlook it at all. The function of an arrestor is to
> protect sensitive (and expensive) electronics from a strike. ALL IT DOES
> IS
> SHORT THE COAX. That remote switch is neither sensitive nor expensive.
> Although, as K8RI noted, it's easy to do that with the switch, the coax
> isn't
> connected to the shack, so it doesn't matter. So I'm sticking with my (and
> K1TTT's) advice. Arrestors go as close as practical to the sensitive
> electronics that they protect. Anywhere else is a waste of money.
>
> 73, Jim K9YC
>
Agreed that protection devices should be at the shack entrance. But further
to that they should be located on your SINGLE POINT GROUND PANEL at the
shack entrance. The whole idea is to make sure that ALL lines entering the
shack are at the same potential during a strike.
If there is any distance between the single point ground panel and any other
lines entering the shack then your single point ground system is
compromised.
The reason that the coax protectors (polyphasers) should be at the single
point ground panel (shack entrance) is because any appreciable run of coax
can have current induced onto the CENTER conductor of the cable when there
is high current induced or carried on the outer shield of the cable.
Inducing current onto the center conductor of coax cable may sound strange
but that is what happens. For non believers a simple google of the term
"transfer impedance" will explain how it happens.
The whole purpose of a coax protector such as a Polyphaser is as Jim said
"to short the coax". We can easily ground the shield of the coax with
suitable clamps or bulkhead connectors. No Polyphaser device is necessary to
do this. But to remove energy from the center conductor the device is needed
unless the center conductor is grounded AT THE ENTRANCE PANEL.
73
Gary K4FMX
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|