On 9/8/2011 12:24 PM, Dave Platt wrote:
In ,
Jim wrote:

Well.. Consider a 20 meter long wire hanging 10 meters above the
ground (half a 80m dipole) so now you're talking tens of pA, 100 Meg
isolation isn't hard to get with clean insulators. That 20 m wire is
like a 850 pF capacitor. If assume, say, 25 pA charging current, you
get a volt every 42 seconds. After half an hour or so, you're up to 40
volts or so..

Granted it's not a lot of Joules.. (heck, probably not even a microjoule)..

So fair weather charging isn't likely to kill your MFJ..

Back to the dust/rain/induction charging, which can certainly get the
levels needed.

I've observed clear-air charge on a TV antenna (about 30' above ground
level) which was sufficient to create a visible spark when I touched
the center connector of the coax to a grounded static-drain block.

No rain at the time, no appreciable amount of dust (SF Bay area)...
just clear air with a mild breeze.


There could actually be a fair amount of particulates without you being
aware of it. I used to work for a company that made things that created
fine particles in air and we tried to remove the particles as well
(often using electrostatic techniques).

For very small particles, they don't scatter the light very well so you
don't see them. Ultimately, you get small enough: molecule sized and you
get blue sky, but even fairly large particles (on the order of a
wavelength of light in size) are basically invisible as such, but do
increase the attenuation, and do scatter some light so you get
decreasing visual contrast at long distances (called atmospheric
perspective in the computer graphics biz) One way that cinematographers
can make a room seem bigger is by putting a very small amount of haze
into the room, because that makes distant things less contrasty, which
your brain turns into "farther away".

Some numbers:

At 1 mg/cubic meter of 1 micron diameter particles, you won't see any
noticeable haziness. That's about 2E15 particles per cubic meter, and
each one can have a max charge of about 1E-13 coulomb. So you can
potentially move 200 coulombs with a cubic meter of air. That gentle
zephyr blowing by at 1 m/sec can move a lot of cubic meters in a fairly
short time. Say you have a 1cm diameter antenna element.. it will
intercept 0.01 cubic meters/sec of that breeze.. 2 Coulombs/sec - 2
Amperes...

In reality, of course, the particles won't have that much charge
(they'll repel each other, for one thing), and not all of the particles
will touch your antenna, especially once it starts to charge, and so
forth. When I was working with a Vollrath Electrostatic Charging scheme
(like a Van de Graaff generator using dust instead of a belt) the best
kind of charging current I got was 10-100 microamps with dust flowing in
a 4" diameter pipe at several meters/sec.

But, the real thing is that you can get particulate charging in what
seems to be perfectly clear air.

Compressed air system run into this kind of thing... you can have fine
oil particles from the compressor, or condensed water particles from a
sudden expansion of the compressed air, both of which might be
invisible, but which can build up a surprising charge.