|To:||<email@example.com>,"'Tower Talk List'" <firstname.lastname@example.org>|
|Subject:||Re: [TowerTalk] Conductive Concrete and Grounding|
|From:||"K8RI on Tower Talk" <email@example.com>|
|Date:||Thu, 27 Jan 2005 15:18:33 -0500|
I think of lightning as a poor square wave. It has an fairly abrupt rise time and a bit slower fall time as I recall.
It's also complex, with changes in amplitude and usually consists of multiple closely spaced strikes appearing as one flash that might flicker a bit. It may even have several distinct separate flashes.
Even if the stroke were always in the same direction the rapid varying amplitude would make it basically an AC signal. If you pick the mean current and then measure either side you will see substantial voltage swings which would be positive and negative in reference to that point.
Taking the square wave of short duration. Tom remembers this stuff much better than I so he may need to expand (or correct).
The theory part is a tad confusing as a perfect square wave consists of an infinite series of harmonics. If that sounds confusing you should try to figure the band width of a network signal which is basically DC. Yet, it's DC only in the sense that it stays positive (I believe it's positive) in reference to the common, or return path. The faster the rise time, or fall time the broader the signal. Remember even CW is not zero band width but depends on the sending speed as well as the characters being sent.
The power for the perfect square wave would be a summation of an infinite series, but in real life the lightening is a far cry from a perfect square wave. In that case the power is basically a summation with some limit and the power drops off at a given rate with frequency.
It's that rapid rise time that sometimes causes lightning to strike a tower and then get off part way down.
The magnetic field is so intense the current takes a different path. I've seen guy wires on a commercial tower glow blue with the fire but it did no damage.
Roger Halstead (K8RI, EN73 & ARRL Life Member) N833R, World's Oldest Debonair (S# CD-2) www.rogerhalstead.com
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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