[TowerTalk] Rain Induced Voltage

[Jim Lux]

At 04:47 AM 12/1/2006, Dennis OConnor wrote:
>This morning in Michigan we are having a mix of sleet and freezing 
>rain... I was up early so I wandered out to the shack about 5AM to 
>see if any Pacific DX was coming in on 80 or 160...  When I 
>energized the antenna switch I was greeted by the sound of a 
>frying/popping noise at about 4-5 cycles per second and an S-meter 
>completely pinned to the right...  I know that sound quite well and 
>instantly cut the power to the antenna switching box...  I unscrewed 
>the coax to the one antenna that is not link coupled - meaning I 
>have a DC connection to the actual antenna - a 160 meter 
>dipole...  Using the tip of my pocket knife I can draw an arc 
>between the shell and the center pin on the PL-259... The VOM reads 
>from 5 to 7 volts DC across the connector continuous into a 1 megohm 
>load...  There is no convective activity in the area, no thunder or 
>lightning, and no thunder storms predicted...  The voltage appears 
>to be induced by the rain drops dripping off the antenna wire removing
>  electrons and leaving a static charge with the  ungrounded side of 
> the antenna <coax center pin) left positively charged...

The mechanism you propose (or one similar to it) is very likely 
what's going on.

Check out the Kelvin Water Dropper electrostatic generator.
http://en.wikipedia.org/wiki/Kelvin_water_dropper

FWIW, the raindrops probably are charged, even without a thunderstorm 
around.  One of the mechanisms by which that charging occurs has to 
do with the transition between water and ice.  And, there's always an 
electric field of around 1kV/meter (fair weather field).  But, as you 
have a dipole, both sides of it would be expected to charge equally 
(at least at the antenna).  Then, as you note, one side is grounded, 
the other not.

And, a related phenomenon, P-static, occurs in planes just flying 
through rain, without the need for thunderstorms.

>I know we have genteely discussed this before <called a scrum in 
>some circles>, but since it is fresh in my mind, I thought I would 
>bring it up again <also known as stirring the pot>...
>
>denny / k8do