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Jim Lux · July 23rd 10, 10:23 PM posted to rec.radio.amateur.antenna

wrote:
On Jul 23, 3:48 am, "Szczepan Bialek" wrote:

You can not eliminate a direct strike by bleeding off the charge.
You can only offer it a better and easier streaming target than
whatever you do not want struck. And the sharper and more
pointy an object, the better it streams. When is the last time
you saw a lightning rod with a polished ball on top?

Actually, air terminals with blunt ends and sphere ends have been tested
by Erico, and they work about the same as the traditional "Franklin"
style with the pointy end.

They don't sell them, as they would be fairly useless.

That's not because they work or don't work, it's because fashion
dictates pointy end. You'd have to have a substantial performance
improvement to justify something different (i.e. to be unfashionable, it
had better give you something else).

At the fields in play here, whether you have a 1/10" radius or a 1/2"
radius or 3" radius, it doesn't make much difference.

the breakdown voltage for a curved surface is roughly 3MV/meter or
70kV/inch.

So a 1/2" diameter rod (1/4" radius at largest) will breakdown at about
17-18 kV. A 6" diameter ball breaks down at 200kV or so (assuming it's
perfectly smooth, etc.)

The "free air" electric field without a storm around is on the order of
100 V/meter, rising to 10-20 kv/m under a storm. So, get 10-15 meters
up, and you've got your 200kV needed to start corona on even a pretty
big diameter thing.

If the ball is wet, especially with distinct droplets, then you can get
corona forming much earlier. The electrostatic forces tend to make the
droplets fly off.