[TowerTalk] Lightning Protection Question

Gene Smar ersmar at verizon.net
Sat Sep 11 17:36:29 PDT 2010


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 at audiosystemsgroup.com>
To: <towertalk at 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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