[Towertalk] Feedlines inside or out?

[Bill Hider (N3RR)]

I'll try keep it short:

The amateur radio "rule of thumb" is as I have stated in my post - Using a
ground kit (or equivalent), ground the coax  to the tower at the top (or
close to the antenna) and at bottom of the tower.  See: www.erols.com/n3rr
for examples/pictures, etc.

PolyPhaser explains all this in detail in their book: "Lightning Protection
& Grounding Solutions for Communications Sites", First Edition, Ken R. Rand,
Published by PolyPhaser, 2000.  Polyphaser's previous editions of "The
Grounds for Lightning Protection" explains this as well.

Briefly, if an *antenna* is hit by lightning, you want to shunt the strike
to the tower ASAP and NOT let the strike run down the length of coax,
insulated from the tower.  That's why you need to ground the coax at the
top.  If your tower is 150 feet tall or shorter, you then only need to
ground the coax at the top and bottom of the tower.  PolyPhaser explains why
the L(di/dt) does not build up too high over the 150 ft run (and why
additional ground kits are not required in this 150 ft run).

For antennas above the 150 ft point, you'll need additional ground kits
every 75 feet to no more than 100 ft spacing.

But, since most ham towers are less than 150 tall, the rule of thumb
applies.  Even at 200 feet, applying the rule of thumb is not a "bad"
practice.

Hope this isn't too "hand waving".  It's a short explanation of an extensive
subject that I have studied for many years.

Bill, N3RR



----- Original Message -----
From: <kb9cry@attbi.com>
To: "Bill Hider (N3RR)" <n3rr@erols.com>
Cc: "Steve/n0tu" <n0tu@codenet.net>; "TowerTalk" <towertalk@contesting.com>
Sent: Wednesday, May 29, 2002 11:30 AM
Subject: Re: [Towertalk] Feedlines inside or out?


> Please 'splain to me why coax shields should be grounded
> at the top of the tower in addition to at the bottom.  I
> understand at the bottom, to shunt the voltage to the
> tower and its grounding grid, but don't see any
> advantage to also doing it at the top unless one wants
> to possibly save some coax once it gets burned up by a
> strike.  Phil  KB9CRY
> _______________________________________________
> Towertalk mailing list
> Towertalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk



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