[TowerTalk] Rain Static?

Jim Lux jimlux at earthlink.net
Wed Dec 27 11:10:06 EST 2006

>So, all of the above is presented to ask the question:  Does what I describe
>accurately depict what one might find in a rain static situation?

Yes.  Especially the cold temperatures.  The freeze thaw transition 
greatly increases the charging of the droplets (while it was liquid 
down where you are, odds are clouod droplets are frozen up a bit 
higher).  Liquid drops, when highly charged, tend to break themselves 
apart into smaller droplets (a phenomenon used to great advantage in 
electrospray devices).  Solid particles, on the other hand, can get 
much higher per particle charges (limited only by the breakdown of 
the air around the particle).

There's also some other interesting stuff going on as it freezes 
(That "electret" in the mic element is the electrostatic equivalent 
of a permanent magnet, and one way of making them is to charge a 
liquid insulator and let it solidify)

The classic p-static/rain static is a phenomenon where small charges 
accumulate on the antenna or surrounding structures, and periodically 
discharge into the air or other structures.  You can set up a couple 
small spark gaps (or neon bulbs) and a couple caps and resistors and 
get a siren like phenomenon where you get a rising pitch that 
suddenly drops.  Same thing happens with antennas.  Or, you just get 
background crackling with periodic big "pops".  It's all the little 
things discharging a lot, and a big thing discharging more frequently.

It's more like corona discharge, actually.

St. Elmo's fire is another possibility, but it's a sort of different 
phenomonenon, and requires a strong E-field and liquid film of water 
on the structure.  I know someone doing some research on St. Elmo's, 
and I'll have to ask if they've looked at the RF emissions.

The actual charge on a single raindrop is fairly small, so the energy 
transfer "per drop" is small, and it's not clear whether you could 
detect it with a radio receiver. (mostly because the impulse when it 
"connects" with the antenna is very short time duration, so the RF 
energy is spread out over a wide band, so you don't get very much in 
any arbitrary 2kHz chunk)  With a moderately specialized detector, 
it's fairly easy to detect and measure the charge on single 
raindrops. (Charge amp with a metal tube inside another, and really 
good low noise amplifiers)

>And, if I
>may ask a follow-up question:  I've had this old CC yagi up for many years -
>it's probably well into it's 25th year.  I've never heard anything like this
>RFI before...from any antenna, much less, this particular array.  What might
>have changed, d'ya think?  Oh - and, the antenna 'plays' as well as it ever
>has - no changes in the measured parameters, at least from the rig end of
>the coax cable.

Probably some incidental thing.  Where the rough spots are on the 
antenna or support structure, for instance.  Maybe there's some 
corrosion between two components that makes a slight insulator, 
allowing differential charging, with periodic breakdowns through or 
arround the corrosion.

Jim, W6RMK 

More information about the TowerTalk mailing list