Towertalk is great! I also appreciated Jim's (W6RMK) response. I had not
read a static explanation like that before.
John KK9A
Michael Tope W4EF wrote:
This is an excellent explanation, Jim. I was hoping you would chime in
and help clarify the confusion. I've read the arguments about why
grounding Yagi elements won't help with rain static, but I always found
the explanations wanting. Yours is a very clear explanation of the
distinction between that phenomenon and the kind of static buildup that
Jack W6FB was describing.
If it isn't already in there, this would be something useful to include
in the ARRL Handbook or ARRL Antenna Book.
73, Mike W4EF.......................
On 12/17/2025 9:44 AM, Jim Lux wrote:
>
> Enlarging on this a bit - They are all examples of charging by particles
hitting or coming near something. Whether it's wind blown dust, snow, or
ice, or raindrops, etc. And whether the charge got on the particles by
induction, tribocharging, or other sources.
>
> There's two notorious effects - one is that "the antenna gets a charge" -
which a static dissipation choke/resistor/whatever will fix. Same for open
telegraph or telephone lines. one of the spectacular forms of this is the
"glowing tips" of helicopters in dust storms, but it also occurs on airplane
props, etc. And, of course, charge accumulating on the airplane body. In
planes, you discharge it with sharp points to force corona to form and bleed
off the charge gradually.
>
> The other effect is where *something other than the antenna* gets charged,
and periodically it breaks down with a spark or corona brush, discharging
it. Basically a relaxation oscillator. That's the one that makes the buzz
or whine. Since the spark is wideband, you're hearing it in your receiver
as a sort of discrete noise source. Obviously, nothing you do to your
antenna will do anything about that.
>
> There is an interesting case reported (I'll have to look for it) where
there were two structures that would get charged, and the charge interacted,
so that caused a changing frequency of discharges (since the relative
voltage on the two objects affected the charging rate in the relaxation
oscillator, which changed the rep rate of discharges). You can create
something like this artificially with a Van deGraaff generator and some
suitable spheres (to hold charge) and spark gaps. Or, with a machine that
unspools plastic sheeting from one roll to another (to make precut lengths)
- That one was quite impressive, because the charging current was impressive
(milliamps).
>
> I'm not sure how you'd fix this kind - unless you have control over the
structure AND there's a way to dissipate the static in a "non-periodic" way
(i.e. have lots of sharp points, to turn it into a steady hiss of corona -
e.g. christmas tinsel on the sheet rolling machines).
>
> Interestingly enough St. Elmo's fire isn't actually corona from the
objects. It's a bit more complex - what happens is that the object is wet,
and the field builds up creating electrostatic forces that tend to "spray"
particles off the tip. Those particles are charged, often with sufficient
charge that the surface field is > 3MV/m breakdown of air, so it's the
particles that are glowing, not the object. The other thing is that charged
water droplets start to evaporate, so the radius gets smaller, but the
charge remains the same, so the surface field rises, leading to corona at
the particle surface. Salt water works better than fresh. You can set up a
demonstration if you have something like a 50kV DC supply - put a tray of
water with a cork or piece of wooden dowel standing up in the tray that's
wet. Put a flat electrode above the cork (at least an inch or so, you don't
want regular breakdown), and connect the power supply between tray and
electrode. Styrofoam cups work well as supports for the upper tray (which
needs a rolled edge so you don't just get corona off the edge of the tray- I
use foam covered by aluminum foil tape for this kind of thing) In a dark
room, you'll see St. Elmo's fire surrounding the top of of the cork - it's
pretty impressive and it "looks different" than regular old corona. Corona
tends to have a sort of branched fuzzyness that is little leaders coming off
discrete points, while St. Elmo's is more of a glowing cloud. Hard to
describe, and I've not been able to get a good picture of it, but once you
see it you'll know they're different.
>
> BTW, there is a theory that dust devils on Mars glow from corona discharge
from the charged dust particles. There is a famous paper by Renno that
reports glowing sand blowing off the tops of dunes at White Sands National
Park. I think there was recently some sound recordings that seem to confirm
this, but it's not imageable with the current sensors.
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