[TowerTalk] Ground Radials Insulated or Not

[Gary Schafer]

Jim Lux wrote:
> 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?
> 
> 
> 
> 

#6 wire is often recommended to carry the current to ground.
#10 will carry 2.5 times less.
#18 will carry 6.4 times less than #10.
#18 will carry 16 times less than #6.

So if you were to use #18 for your radials, 16 of them would have the 
current carrying capacity of a single #6 wire.
Yes the current does divide pretty much equally in the radials.

It is even better with radials than just parallel wires as the radials 
afford more dissipation to ground being spread out. The ground does not 
get a chance to saturate as it can with only one or a few ground rods.

A fact is that "a good lightning ground makes a good rf ground", "but a 
good rf ground does not always make a good lightning ground". (as in 
elevated radials)

We know that just ground rods do not make a good rf ground in most 
cases. They don't make a good lightning ground either.


Lightning propagates just like rf. It takes time to dump all the energy. 
If you try to do it all at one point the ground saturates and the 
voltage will rise high. With a radial system it allows the energy to 
dissipate as it travels. Radials are lossy transmission lines.

If you are uncomfortable with using only the buried radials for a 
lightning ground then attach some ground rods also.

In a common lightning ground system installation it is recommended that 
ground rods be placed around the tower and separate radials run out to 
each ground rod from the tower. Additional ground rods would be 
installed at approximately the distance of twice their length on each 
radial to the same wire.

You want as many connections and directions from the tower that are 
practical. I.e. a radial system.

A ground rod is really a radial in itself. It runs down rather than 
parallel to the earth.

Am broadcast stations depend on the radial system for lightning grounds. 
In some cases where soil conditions are poor it has been found that 
adding ground rods at distances along some of the radials helps.

If you were to use ground rods along some of the radials you would want 
to use heavier wire for those radials rather than #18 or so. But you 
don't need to go the #6 if you have a large number of radials as the 
current is going to be divided in all runs.

73
Gary  K4FMX