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Re: [TowerTalk] Ground Radials Insulated or Not

To: Jim Lux <jimlux@earthlink.net>
Subject: Re: [TowerTalk] Ground Radials Insulated or Not
From: Gary Schafer <garyschafer@comcast.net>
Reply-to: garyschafer@comcast.net
Date: Mon, 06 Dec 2004 13:46:01 -0500
List-post: <mailto:towertalk@contesting.com>


Jim Lux wrote:
At 08:18 AM 12/6/2004 -0800, Michael Tope wrote:

From a performance point of view, I suspect it doesn't make
any difference at all whether the wires are insulated or not.
One consideration that I haven't heard discussed with regard
to radials, however,  is lightning protection. Wouldn't it be better
from a lightning perspective to have at least some of the radials
in the system uninsulated? Perhaps a few heavy radials (6 to 8)
made from #4 bare copper interspersed with the balance of the
radial system made from whatever is cheapest. Just a thought.

Mike, W4EF......................................


One might argue that you want to keep the "RF grounding" function of the antenna (which, after all, is connected to the coax shield) separate from the "lightning current discharge" function.

How are you going to keep them separate?



The junction from antenna/tower/whathaveyou to the lightning ground is going to rise in voltage pretty substantially, regardless of how good the ground is. A notional 5 ohm ground with 10kA lightning stroke current is going to go to 50 kV.

That's right! The lightning is not going to discriminate between anything that you want to call a lightning ground and the radials that are in the ground that you call an rf ground.


Why not take advantage of all that work of putting in radials. That is one of the best lightning grounds you can get. It provides many paths for the lightning to dissipate. Much better than several ground rods.

However it would not hurt to install a few ground rods connected to the radials in addition.


I'll have to look for the details, but I seem to recall that one of the analyses that NEC-4/NEC-3 were designed for was to look at whether lightning grounds should be combined with RF grounds (particularly radial fields). I do recall that they came up with the surprising finding that with elevated radials, the performance with the radial field connected to the ground stake was worse than with them separate.



That would make sense as an elevated radial system is done to isolate it from ground losses.


73
Gary  K4FMX

Jim, W6RMK
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See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.

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