> ORIGINAL MESSAGE:
> On Tue, 11 Jul 2006 11:55:08 -0700, you wrote:
>>Yes. If you think of lightning as DC, you're likely to be in
>>serious trouble. IEEE studies show that the energy content in
>>lightning has a broad peak around 1 MHz, with significant
>>content well above and below that range.
> ------------ REPLY SEPARATOR ------------
> That raises an interesting question. As I understand it, lightning
> really does flow in one direction, making it DC but having a
> square-wave nature. Is that where the HF component comes from?
> Lightning doesn't really change directions, does it?
Lightning is a complicated and unpredictable phenomena.
Yes, it's essentially DC...pretty much, almost, sorta, but:
There are both negative and positive strikes. The so called super strikes
are thought to be positive and quite likely associated with the "Sprites"
the astronauts have seen above the clouds.
However (IIRC), As has already been discussed lightning strike/bolt is
preceeded by a step feeder making a connection. This in itself is a small
strike followed by the main strike of the opposite direction. There then
follows a series of interactions between cloud and ground. However the
source of the HF we hear is created by the rise and fall times of the
bolt(s). Most lightning strikes consist of (I believe the number is) 3 to 5
and possibly as many as 10 closely spaced strikes that appear as one flash
that "flickers" a bit. the much lower frequency of the multiple cloud to
ground interactions durring the strike is probably the source of what are
called "Whistlers", or very low frequency signals that tend to change
frequency, or sweep across a range.
This very rapid rise time means it contains a range of frequencies. The
rapid rise time coupled with the large current mean very high reverse EMF in
what they strike as well as in the bold itself. Often the rise time is
short enough and the current high enough when striking a tower the lightning
will find an easier path to ground by getting off the tower part way down.
I lived within a few hundred yards of a tower on top of a tall hill that was
regularly hit and saw this phenomena a number of times.
Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
> Bill, W6WRT
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