"chuck" wrote in message
...
Jim Kelley wrote:
chuck wrote:
Hi Jim,

Can you elaborate further on your understanding of just how an
ungrounded antenna wire that receives an increment of charge causes a
response in the receiver as the charge is being acquired?

Hi Chuck - given a large enough increment of charge it's pretty
straightforward. Best example I can think of is a capacitor. Take a
high gain amplifier with an input coupling capacitor and start daubing
clumps of charge onto the unconnected input lead. When the amount of
charge is big enough, you'll hear the pop that results from the
instantaneous change in charge. Thousands of those per second and pretty
soon we're talking some real noise. ;-)

If dust particles are charged as raindrops are (mixed polarity and
magnitude) a typical incremental charge is on the order of 8
picocoulomb. Do we have an understanding of how much power would be
transmitted to the 50 ohm input impedance of the receiver by such an
incremental charge and the mechanism by which that transmission takes
place? Based on anecdotal info, it is not unusual for these impulses to
appear at an average level of several S-units.

Thanks for any insights you can share.

Chuck, NT3G

I've never worked these numbers before, but given your typical charge per
particle of 8 picocoulombs, and a 16uV signal from a 50 ohm antenna
(converted to uamps), I get about 4x10^4 of those particles per second.
Given the density of air, that's a pretty low number I think. If I'm
wrong, please don't blast me. The back of my envelope here is pretty
messy and hard to read. :-)

Hi Jim,

Seems I misunderstood your original post as suggesting that each
incremental charge produced a pop.

I see that the typical current density of rain in a storm is on the order
of 0.1 - 1 uA/square meter. Taking the usual obscene liberties with
priors, that's on the order of 60 - 240 pA for a 40 meter long wire with a
diameter of 2 mm.

The average voltage developed at the receiver front end (50 ohms)would
then be on the order of 12 nanovolts (240 pA x 50 ohms). This is obviously
many orders of magnitude less than reported p-static signal levels.

Using your capacitor-coupled amplifier example, a continuous, average
current of 240 pA would be more like DC than what we seem to hear. I am
persuaded that there may be some kind of relaxation process going on in
which the charges build to some breakdown level and appear as a pulse
(significantly greater than 12 nV) at the receiver input. But what breaks
down? Diodes? Presumably we are not talking about arcing here. Does anyone
recall this phenomenon in the vacuum tube receiver era?

Also unknown is the state of charge of the ungrounded antenna wire just
prior to the acquisition of an incremental charge. Is the antenna at
some high positive or negative potential just prior to impact, allowing a
small incremental charge to cause a breakdown?

What I think we do know is that there are sometimes charged particles
(dust, rain, snow, etc.) that impact an antenna and cause a characteristic
type of noise in the receiver. Exactly which particles, with which
charges, and under what conditions of pre-existing charge in the antenna
do we see such noise and at what signal levels?

I'm sort of at the analogous point of noting that crop growth is
correlated with some periodic level of rainfall, without a clue as to
the mechanism by which the raindrops and plant growth are related. ;-)


the noise is caused by corona on the antenna due to the electric field
between the ground and cloud. this field can be many thousands of kv per
meter which is enough to cause sharp points and tips of elements to generate
corona which makes the hiss and pop noises.