Lightning is a complex waveform. Typical rise times can be 1-2
microseconds with tails that decay from 10 to 100's of microseconds. A
sine wave with a 1 microsecond rise time (1/4 of the cycle) would be
4MHz, significant frequency components obviously go well above that but
get weaker as you go up. As a quick example compare the strength of
static from a thunderstorm say 50 miles away... the noise on 80m will be
much louder than on 10m and up into the vhf bands it gets progressively
weaker.
However! Lightning can have a significant dc component also. As in the
stroke that w8ji commented on that melted coax shields and other wires
and magnetized CRTs. Large positive strokes can have peak currents of
100-300ka. But even worse, the tail of the stroke can last milliseconds
at levels of hundreds of amps. These are the strokes that burn down
power line shield wires, melt holes in steel, and do all sorts of nasty
things. To point out the difference between the potential power of
these strokes consider this... where I work we have a pulse generator
that can put out 30-40ka pulses a few microseconds long, we use it for
testing insulators, soil characteristics, lightning arresters, and other
odd things... I have seen those pulses carried by 12ga copper wire with
no visible damage. Put that same 12ga wire across a battery and in
milliseconds it is melted.
David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://www.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Pete Smith
> Sent: Thursday, July 29, 2004 20:18
> To: Jim Brown; Tower Talk List
> Subject: RE: [TowerTalk] Grounding, Lightning & corona discharge
>
> At 03:57 PM 7/29/2004, Jim Brown wrote:
> >When thinking about it, remember that the energy in lightning is NOT
at
> >dc, it is around 1 MHz. The impedance of that path is primarily
> >INDUCTIVE REACTANCE, not resistance. Resistance (greatly
> >increased by skin effect) will certainly produce the I2R losses,
but XL
> >will determine current!
>
>
> I recall reading somewhere (I thought in Polyphaser material but can't
> find
> it there now, in a quick search) that there is meaningful energy in a
> typical lightning bolt that is effectively far higher frequency than
that
> -- somewhere in the 100 MHz range -- because of the fast rise time of
a
> typical strike. This was in connection with advice that ground wiring
> follow VHF precepts in being as straight and wide as possible to
> absolutely
> minimize inductance.
>
> Fact or fiction? It would be a lot easier for me to improve my
protective
> grounding if I could use copper strip but have a couple of right-angle
> bends in it.
>
>
> 73, Pete N4ZR
> The World HF Contest Station Database
> was updated on June 5, 2004
> 2728 contest stations at
> www.pvrc.org/WCSD/WCSDsearch.htm
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers",
"Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041
with
> any questions and ask for Sherman, W2FLA.
>
> _______________________________________________
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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