[TowerTalk] Rain Static?
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.
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