[TowerTalk] Rain static

[David Robbins K1TTT]

> >
> >The percipitation noise that I have heard has always been broadband
> >RF impulses (e.g. pitch doesn't change with VFO frequency).
> 
> 
> That would be consistent with charged drops (or snowflakes or dust
> particles) that hit the "thing" and steadily deposit charge.  The voltage
> builds up, and periodically discharges.

Not really.  If it were charge from drops it would spread out down the
conductive structure of grounded elements and bleed off quickly that way.
This would also not explain clear air 'precipitation' static that occurs
before or after the rain or snow is hitting the antenna.  It would also not
explain why lower antennas do not have as bad a noise as higher ones.


> 
> In airplanes (which use AM radios), this is the general mechanism for P-
> static.
> The "static dissipating wicks" you see at the ends of the wings are an
> attempt to reduce the problem by making many small discharges, a long way
> from the radio antenna, rather than periodic big discharges.

Airplanes are different than towers.  Airplanes are not grounded and they
hit many more drops that are in a highly charged part of the cloud.  Towers
and antennas are grounded, don't move through the precipitation, and are
much taller.  It is the vertical component that makes them simple 'short
circuits' across the sometimes very large electric field gradient near the
ground.  This gradient can be 10kv/m or more under a charged cloud even
without precipitation, so over a 30m tall tower you can have 300kv voltage
difference from top to bottom, much more than enough to cause severe corona
from any object on top of the tower.  Lower down on the tower the voltage is
proportionally lower, and is locally even less because of the conductive
umbrella over it.  Do some modeling with a good electric field modeler like
Ansoft as I used to do where I used to work and you will see this.

> 
> There are people who actually build toys to measure this kind of think.  A
> wire outside connected to a NE2 neon bulb with the other terminal of the
> NE2 grounded.  It blinks periodically, with the blink rate proportional to
> the charging rate.
> 

there was a project in one of the old science or electronics magazines back
in the 60's or so I think that ran a motor off the clear air electric field
gradient.  And there are stories of Franklin building a lightning warning
system with a bell and suspended ball that was connected to a wire outside,
as the ball charged it was attracted to the grounded bell and rang to warn
of approaching storms.  Even without a storm nearby there is a gradient of
(IIRC) several hundred volts per meter... as a storm approaches this can go
up quickly and produce the much larger potentials that cause us problems.  

David Robbins K1TTT
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