Re: Lightning Detector

[William Beaty <billb@eskimo.com>]

On Wed, 5 Jun 1996, Dave Simmons wrote:

> I just received an ad enclosed in my electricity bill for the
> "Guardian Angel" lightning detector. It is a hand-held, battery-
> powered device that is supposed to detect the approach of storms by "
> analyzing electromagnetic frequencies in the air." It has LEDs which 
> light up to give the distance to the storm/lightning up to 40 miles 
> away. The ad suggests taking it "while you golf, picnic, boat...It 
> might just save your life." 


Off the top of my head, I'd suspect that the device receives magnetic
impulses via a coil, then analyzes the waveforms.  Very near a lightning
strike the field is going to be proportional to the current.  Far away,
the field will take the form of propagating EM, and the wave will be the
second derivative of the current and will have several positive/negative
excursions. If the waveform of lightning current is fairly similar from
stroke to stroke, and if the length of lightning bolts is similar, then it
would be easy to figure out the distance by looking at the shape of the
waveform.

But for $200 this seems a bit much.  Maybe the device just "listens" to
the VLF band and judges storm distance by the intensity of "sferics," the
constant pulses which originate from small lightning (length scale in tens
of feet and below.)

I looked in Martin Uman's LIGHTNING (1984, Dover) and it describes the
variation of e-field with distance from the lightning, but doesn't go into
detail on b-field.  Unless your detector has a long telescoping antenna,
it's bound to have a ferrite core coil as an antenna.

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