[TowerTalk] Ground Radials Insulated or Not

[Jim Lux]

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?