I think that the pstatic doesn't come from the charging event (the snow
flake or dust) but from the discharge events when the conductor is
sufficiently charged to discharge. The energy/charge in a single raindrop,
snowflake, etc. is just too low. It's probably in the area of one or two
electrons per particle.
The usual impact charging mechanism (dust, steam, etc.) is that the
particles provide a fairly constant current input into the object being
Velocity doesn't have much effect on the charging, other than to increase
the maximum potential current, although there are some conflicting effects
that reduce that.
For purposes of illustration, consider a rain drop 0.1mm in diameter. It
will have a capacitance (isolated sphere in free space) of about .01 pF, and
a limit voltage (assuming 30kV/cm) of 150V. That's a stored energy of
1E-10 Joule, and a charge of 1.5 picoCoulomb.
make the drops 1mm in diameter, and the capacitance is now 0.1 pF, and the
limiting voltage 1500V. The max charge is now 150 pC (since max charge goes
as the surface area, of course). The stored energy, though, is still quite
small, around 0.1 microJoule.
Snow flakes are big, but have sharp edges, and carry as much charge.
I just don't think that most receivers can detect these microjoule events.
The charging impulse is going to be fairly short duration and broadband, so
the energy is spread over a fairly wide band. (no question that if it were a
microjoule in a narrow band it would be detectable)
On the other hand, a discharge event, from a 10 meter long rod 3 cm in
diameter (C of several thousand pF, charged to a few kV) is a fairly high
energy event (say, a few milliJoules), and, because it's from a resonant
structure, might well be narrower band (the travelling wave of the current
feeding the discharge will propagate along the element).
For comparison, a 10dB NF in 1kHz BW is -154 dBm, or 4E-13
There's a fairly complete literature on electrostatic charging by dust, etc.
Vollrath is one author to look for (he made a Van deGraaff type apparatus
using blowing dust as the charge carrier, rather than a belt). There are
also reports in the literature of 3 foot sparks from the tops of sand dunes
at White Sands National Monument apparently from windblown dust charging.
And, of course, there is an enormous amount of literature when one starts
looking at electrostatic dust precipitators.
----- Original Message -----
Sent: Tuesday, August 31, 2004 2:57 PM
Subject: [TowerTalk] upper yagi noise--a reason?
> I have a theory....
> w8ji wrote:
> Snow would have an entirely different noise than heavy rain
> drops, and dust in wind would be an almost perfectly smooth
> white-noise hiss. It would all tie directly in to the
> particles per second hitting the antenna if it was charge
> movement between the antenna and the media striking the
> antenna. If I had 60 large drops per second hitting the
> antenna it would be a strong 60 Hz pulse. If I had 5000
> gentle snowflakes, it would be a 5000Hz low level pulse.
> The frequency would NOT change unless the contact rate
> changed since the pulse rate would very clearly be synced to
> the rate of the media contacting the antenna. Only the
> intensity (level) would change as charge gradient varied.
> Yesterday, I observed what a 40 story building did in the backwash of
tropical storm Chuba, typhoon # 16, as it blew across Tokyo yesterday. On
the 21st floor, the building creaked like a sailboat, with about a 4 second
resonance period. First the window wall, then the hallway door frame..back
> When I went out, it was blowing a good 35-40kts on the ground. At up at
the tops of the buildings?
> The answer is, airflow is less affected by friction with objects, trees,
etc. up high, than it is down low. The upper antenna sees more wind,
intercepts more particles of snow, rain, sand, etc. and thus builds more
> I remember taking down a 100' rohn 25 at 2000' in the poconos, some years
ago. At 50', it felt breezy. At 100', my tag line blew out horizontal and
stayed there. I had to add 10 lbs to the bucket, to keep it where the
ground crew could reach it.
> So that's my theory...it builds on Tom's physics discussion, and makes
> 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
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