[TowerTalk] porcupines and such

[Didier Juges <didier@cox.net>]

Bill,

I did not say the charge on the earth was "absolutely" uniform. It is just 
significantly more uniform than that of the clouds because the earth is 
considerably more conductive (I did not say earth was a superconductor 
either) than the clouds, at least until ionisation takes place. Until 
lightning strikes, currents are very low (if I recall, just before the main 
discharge, currents are in the low amperes range), so the resistance of the 
earth (which is measured in ohms, not hundreds of ohms in most cases) has 
little effect, because potentials developed across the earth by these 
currents are measured in volts, not kilovolts. Said another way, the 
probability of lightning is not very dependent on the soil quality (even 
though a VERY poorly conductive soil may affect the probability by raising 
soil resistance to 10s of kilo-ohms possibly), but the effects of lightning 
are. Please note I said IT IS NOT VERY DEPENDANT, I did not say IT IS NOT 
DEPENDANT.

We certainly have generally conductive soil here in Florida, where it's all 
soaked with water, some of it being salty water. Yet, we have lots of 
lightnings.

I am talking about relative effects, not in absolute terms.

Of course the charge in the clouds will attract opposite charge on the 
ground. Please note I equated the clouds and earth to a capacitor, and this 
is how capacitors work (I refer you to your favorite electricity 101 book). 
The point of the posting is that unlike someone said, the porcupines do not 
dissipate the charge in the earth by an appreciable level. Their only 
effect is to reduce the charge IN THE AIR around them. Not quite the same 
thing. I did not say that was useless either, even though my belief is that 
it is not very useful as a means of avoiding lightning strikes.

The presence of charges is not what causes lightning, it is the POTENTIAL 
that matters, and how close the potential differential is sufficient to 
cause ionization. Now, I agree that charges are what causes potential, It 
would be the same thing as saying spending causes you to loose your credit. 
It only causes you to loose your credit if you spend in excess of your 
credit limit.

The charge on earth's side is considerably more uniform than that on the 
clouds. When was the last time you saw lightning between two points ON 
EARTH? Compare that to lightning between clouds, which are common.

There again, you should keep things in perspective, as it is all relative.

You can read the rest of the posting now :-)

73,
Didier KO4BB

At 07:29 PM 3/21/2004, you wrote:

> Didier- IMHO, your first paragraph has such  fundamental errors that the 
> rest cannot be supported. The charge on the earth side is NOT uniform over 
> miles; the earth is not a superconductor and has finite resistance. All 
> common descriptions I have seen showing the charge distributions show both 
> the earth and the clouds with charge, but of opposite sign. This is 
> contrary to your first statement.
> Bill
>
> At 12:07 PM 3/21/2004 -0600, you wrote:
>
> > The problem is that the ground is not charged, the clouds are charged. 
> > Think of the earth and the cloud as both plates of a capacitor, except 
> > that the bottom plate is a few 10 of thousands of miles long and 
> > conductive, and the top plate (the cloud) is a few thousand feet wide, 
> > and basically a charged insulator (each water droplet is charged, but 
> > isolated from its neighbor. The charge on the earth side is for all 
> > practical purposes infinite. You cannot drain it to anywhere because it 
> > always returns to earth.
> >
> > When charges escape the porcupine, they do not make it to the cloud. They 
> > just dissipate in the air and return to ground, so they do not contribute 
> > to reducing the charge in the cloud. I'll agree that they may locally 
> > reduce the field in the air somewhat (as seen from a distance of a few 
> > feet). The argument is whether that reduction is sufficient to reduce the 
> > probability of a lightning coming from hundred or thousands of feet higher up.
> >
> > Think of it another way. The earth is conductive. At least until 
> > lightning strikes causing great amounts of currents, before that point 
> > little current flows, so the potential along the earth is not affected 
> > very much by a few charges flowing out of a porcupine because the earth 
> > is conductive and charges are replaced as soon as they escape. Charges 
> > may be flowing out and into the air, but the earth's potential is the 
> > same, and the potential difference between earth and the cloud is what 
> > causes the lighting to go.
> >
> > The other side of the argument is whether it is actually a good thing to 
> > eliminate smaller strikes, as they act as bleeders and may prevent the 
> > larger strikes.
> >
> > I am not sure I want a lightning protection device that would reduce the 
> > number of smaller strikes at the expense of greater probability of 
> > getting the big one.
> >
> > There is a lot of anecdotic evidence that these types of devices work, 
> > but no serious, objective, peer reviewed studies to support the same.
> >
> > The bottom line, as long as you are happy with it, and you do not cause 
> > other problems doing this, why not do it? Just be aware that it may or 
> > may not work, and it may even increase the probability of getting a 
> > serious hit.
> >
> > 73,
> > Didier KO4BB
> >
> > At 10:20 AM 3/21/2004, you wrote:
> > > I think that's the key.  Their web page is misleading, but, in my own
> > > belief, not entirely wrong.  Yes, there will be direct strikes to the
> > > tower, even the whiskers.  But before the differential reaches the point
> > > of discharge, I think the whiskers are trying hard to discharge it
> > > slowly, thus preventing some of the smaller strikes.
> >
> >
> > _______________________________________________
> >
> > 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.
> >
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk@contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
>
> Bill Aycock - W4BSG
> Woodville, Alabama


_______________________________________________

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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