At 05:08 AM 9/1/2005, David Robbins K1TTT wrote:
> > >
> > >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.
Only if the entire structure really is a solid good conductor. It doesn't
take much power to make an RF audible signal, so what you might be seeing
is charging of some fairly small component that happens to be isolated
enough (maybe that plastic owl?)
>This would also not explain clear air 'precipitation' static that occurs
>before or after the rain or snow is hitting the antenna.
This isn't something I've noticed (but, then, I've not looked). I've
always assumed that increases in atmospheric noise can come from many
sources, not the least of which is numerous microdischarges among charged
particles. It is theorized, for instance, that dust devils actually glow
(very faintly) in the dark, because the charging processes easily get the
voltage on individual dust particles above the breakdown field. There's an
interesting demo where some dust is put into a clear glass sphere or tube,
and then as it's rolled around and around, you see little glows and flashes
(in a real, real dark room, with a bit of dark adaption).
Thunderstorms, even without actual lightning, are pretty good RF noise sources.
> It would also not
>explain why lower antennas do not have as bad a noise as higher ones.
True, it doesn't explain the high/low difference, but then, there are lots
and lots of potential reasons for that phenomenon, most of which aren't
incompatible with the charging scheme I outlined. (Maybe the sparks from
the owl are at the top?)
> >
> > 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.
Actually, I don't think that the highly charged part of the cloud really
enters into it. But, as you say, the problem is much worse for planes
because they are bigger and intercept more drops.
> Towers
>and antennas are grounded, don't move through the precipitation,
wind effectively moves the tower through the precipitation (and, of course,
the precipitation is moving vertically)
> 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.
I'll buy that.. although the tower itself will perturb the field.. That is,
the current flowing in the tower won't be 300kV/tower dc resistance), but,
several mA would certainly not surprise me.
> 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.
Yep.. but while the clear air E field can also cause corona effects, folks
have noticed a very preciptation dependent effect as well.
It's all very, very complex, with lots of mutually interacting
factors. And, of course, of little commercial interest... You just put
protection in the system, make sure corners aren't too sharp, and live with
the noise.
Jim, W6RMK
_______________________________________________
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
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|