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Re: [TowerTalk] Ground Radials Insulated or Not

Subject: Re: [TowerTalk] Ground Radials Insulated or Not
From: Jim Lux <>
Date: Mon, 06 Dec 2004 13:56:49 -0800
List-post: <>
At 01:46 PM 12/6/2004 -0500, Gary Schafer wrote:

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?

You can't necessarily, but it's more that the design requirements for each are different. I don't say it's practical or desirable, but one could imagine a narrow band filter at the feedpoint (connected beween vertical element and ground field) that passes only the desired amateur band.

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.

But is a good RF radial grounding system really "one of the best lightning grounds you can get". A raft of small wires might well be a worse lightning ground than a few nice big wires or rods. The smaller wires may fuse with the lightning current. Say you get a 20kA strike and you've put in 60 wires. That's 300+ amps into each wire (if the current divides equally, which it probably won't). 300 Amps is a ballpark fusing current for AWG10 wire in air (admittedly, that doesn't take into account the short duration of the lightning impulse, etc.)

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

I'll say that this is true. For just the reason described above.

What's standard practice in the broadcast industry?


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