Jim:
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.
As for the need for a final Poly outside my shack, I concluded that I
didn't need one there for a few reasons. The run is short (<50 feet); it is
underground; based on these these two facts it would be nearly impossible
for a lightning strike to induce appreciable energy on the center conductor
of that 50 feet of coax such that I would need another Poly outside my
shack. (We all agree that lightning supressors are intended to shunt the
center conductor to ground during a lightning strike, right?) Any induced
charge on the shield of my underground coax would be grounded at the NEMA
box and at my SPG outside the shack.
As I previously wrote, if I had run the coax directly into the shack to
connect to the switch there, I would have installed Polyphasers outside the
shack on the SPG and not inside the NEMA box. If Larry KN8N has a switch on
the tower, he ought to put the Poly's there. Otherwise, I agree - outside
the shack on the SPG.
73 de
Gene Smar AD3F
----- Original Message -----
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: <towertalk@contesting.com>
Sent: Saturday, September 11, 2010 11:53 AM
Subject: Re: [TowerTalk] Lightning Protection Question
> On Sat, 11 Sep 2010 10:57:06 -0400, Gene Smar wrote:
>
>>It depends. I have my Polyphasers at the NEMA box at the tower base,
>>but I also have a remote coax switch there, too.
>
> I disagree (and agree with K1TTT) -- lightning protection devices are
> protecting you radios by short the voltage on the center conductor to
> the shield. They cannot do that at the tower -- voltage can be induced
> on the coax between the tower and the shack, and a protector at the
> tower is too far away to offer much protection at RF. The best place for
> them is at the entry panel to the shack.
>
> The SHIELDS of coax SHOULD be bonded at top and bottom of the tower.
>
> 73, Jim K9YC
>
>
>
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