[TowerTalk] Rain static

[K4SAV]

Thanks for a very clear explanation, David. This is starting to make a 
lot of sense, and explains different instances of precipitation static.

For stationary towers, the electric field gradient across the tower, 
creating the coronal discharge, makes a lot of sense to me.  It explains 
the data, whereas the charged particle theory does not explain all the 
data (my opinion).

In the case of my mobile rig in the rain, the car is being charged by 
the charged rain drops (just like the airplanes). Which correlates with 
the observation, the faster I go, the worse it gets. (I have observed as 
much as 40dB increase in rain static between 0 and 60 mph.) I'm not sure 
where on the vehicle the largest discharge point would be. Is my vehicle 
discharging to the ground or upward toward the clouds? I would think it 
would be much worse if the point of discharge was from the antenna. 
Maybe it is discharging from many points on the vehicle.

I have a friend who had a similar problem, but occurring without rain.  
He had static which was proportional to vehicle speed (no it was not 
ignition noise) without rain present.  When he stopped the vehicle, 
after a few seconds the static would gradually disappear. At the time, 
it was clear to me that this was a static charge/discharge problem, but 
I was unsure as to what the charging mechanism was.  I was thinking 
maybe it had to do with charge being generated by friction between his 
tires and the pavement. Maybe the charge was being picked up by dust 
particles?  But if it was dust particles, why did not the same thing  
happen to my vehicle?  Any speculation as to the charging mechanism?

So you don't need precipitation to cause "precipitation static".  Seems 
like the name is a misnomer, much like the "single point ground" name, 
although it is unlikely to cause as many problems as the latter.

Jerry, K4SAV

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.
>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
>e-mail: mailto:k1ttt at arrl.net
>web: http://www.k1ttt.net
>AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
>
>
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