[TowerTalk] Rain static

[Jim Lux]

At 04:06 PM 8/31/2005, K8RI on Tower talk wrote:



> >> Does anyone have any ideas of how I can solve this
> > problem?
> >
> > Joe,
> >
> > There is some controversy or difference of opinion on this,
> > but my experience, everything I can measure or simulate, and
> > any data I get from other sources indicates the noise is
> > from corona discharge from sharp points in or around the
> > antenna. This corona discharge is caused by voltage gradient
> > between the earth and clouds around the antenna during
> > inclement weather.
> >
> > The other opinion (that I do not find evidence to support)
> > is that charged droplets or particles hit the antenna and
> > discharge, making noise.

Funny you can't find evidence.  It's widely reported in the scientific 
literature, for at least the last 100 years, and probably longer than that.


>I think you will find that precipitation static  is neither.
>It's basically the a charge building from many droplets, or snow flakes
>removing electrons from the ungrounded elements, or wires.  The charge
>builds to the point where it arcs over.
>
>Snow is particularly bad with this and the stronger the wind the more rapid
>the charge.

It's also been reported with wind-blown dust.  Things such as continuous 
sparks from a long telegraph line to ground.

Pick up any particles off the ground and they'll tend to have a net charge.

A gentleman named Vollrath used the principle to make a form of HV 
generator (sort of like a Van deGraaff, except using dust to carry the 
charge instead of a moving belt).

Phys. Rev. 42, 298–304 (1932)
[Issue 2 – 15 October 1932 ]
A High Voltage Direct Current Generator
Richard E. Vollrath ,University of Southern California, Los Angeles, 
Received 19 August 1932

When powdered materials are blown through metal tubes by means of 
compressed air considerable quantities of electricity are produced by 
contact electrification. It was found that 6 x 10-5 coulombs could be 
produced per gram of diatomaceous earth, a form of silica, blown through a 
short length of copper tube. A generator of extremely high voltage is 
proposed, and a small scale model of such a generator is described, by 
means of which currents of 8 x 10-5 amperes at 260 kilovolts were generated.
©1932 The American Physical Society
URL: http://link.aps.org/abstract/PR/v42/p298

The nice thing about this paper is the analysis of the maximum charge that 
can be carried, etc.

(This paper is reproduced in Ford, "Homemade Lightning", which is available 
in many libraries)

Vollrath cites Petri's 19th century observation that you could draw 1.2 HP 
electrical power from a 5km long wire during a snowstorm. The literature 
reference with the analysis is Ebert and Hoffman,Meteor. Zeits. 317 (1900).

Vollrath also quotes Staeger (A. Stäger, Ann. d. Physik 77, 230 (1925))as 
observing a corona discharge from a 9 meter long wire in a snowstorm along 
with a current of 17-20 mA.  Also mentions a corona discharge 10cm long 
when making CO2 snow from expanding liquid.


Charged dust carried aloft in dust devils has also been proposed as a way 
for charge to get to the upper Martian atmosphere  to support the overall 
atmospheric current that has been observed. (since Mars doesn't have 
thunderstorms, which do that function on earth)

As a practical matter, too, it's of some concern for the Mars rovers.  Not 
only is there a fair amount of dust on Mars, it's real, real dry, so 
there's very little surface leakage to provide a discharge path.  Mars 
atmosphere is about 10 torr, which is very close to the Paschen minimum, so 
it doesn't take much for corona to start.