On 2/20/2015 5:26 PM, Dave Platt wrote:
In article ,
Kurt Stocklmeir wrote:

I would like to know how much am and fm radio waves are made by
lightning

has any person tried this - when lightning is around use their
radio to find how many watts of am and fm radio waves are made by
lighning

thank you for any answers

I think you'll find that your question doesn't "map well" onto how
lightning works.

Saying "AM radio waves" or "FM radio waves" implies that there is a
carrier wave, which is being either amplitude-modulated or
frequency-modulated.

Neither of these is the case for lightning strikes. Lightning
consists of strong, irregular pulses of current with rapid rise
times. Each pulse generates a broadband burst of electromagnetic
energy, with energy content distributed widely across a whole range of
frequencies.

The figure I'm seeing is that a single lightning strike typically
releases about 5 billion joules of energy (5 billion watt-seconds).
Most of this is released in a very short period of time (under a
millisecond).

According to http://www.lightningsafety.com/nlsi_info/thunder2.html
about 90% of the lighting's energy is released as heat, less than 1%
is released as sound, and the rest (call it 10%) is released as
light (and, I presume, other frequencies of electromagnetic energy).
There's a very strong electromagnetic pulse near the lighting stroke
and the earth impact point, due to the high current flow.

In the "radio" frequencies per se (e.g. from a few hundred kHz to a
few hundred MHz) it's probably a fraction of a percent of the total
lightning strike energy... mostly in sharp bursts at the beginning
(and perhaps end) of each individual sub-stroke.


That website is a little bit misleading, probably in the interest of not
confusing the non-technical readers. After all, heat is also
electromagnetic energy

But effectively, lightning is a very broad band pulse - running from a
few kHz (or lower) well into the high electromagnetic range. Strong
strokes are even known to generate gamma rays (and according to recent
theories, antimatter).

But you're correct - the total amount of energy in the RF range (say
3kHz to 30GHz) is pretty small.

As a sidelight - I remember back in the 60's a lightning detector
project. Basically it was a pair of hula hoops; slit each hoop and wrap
them with several turns of small-gauge wire. These fed a pair of
balanced amplifiers which then fed the X and Y axes of a oscilloscope.
A vertical antenna fed an unbalanced amplifier which then fed the Z axis
of the scope to brighten or darken the trace, depending on the phase
(and therefore the direction) of the incoming signal. Properly
calibrated, the result would be a line from the center of the scope in
the direction of the lightning stroke with the length proportional to
the strength (and roughly inversely proportional to the distance) of the
bold. It pretty cheap to build (I was a high schools student, after
all) but a lot of fun to play with.

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