And after all that throw in a few more factors. The near fields, both
magnetic and electric, can do lots of damage also. The magnetic field can
be enough to induce voltages that can flashover power line insulators up to
about 69kv, even if they don't hit the line directly. And the transient
electric fields can do very strange things... think about the reports of
hair standing on end before a nearby stroke, why does that happen?? Because
a large quantity of charge is attracted by the downward moving leader over
the period of maybe a couple seconds. This inrush of charge is relatively
slow, but can be huge... this is followed by the generation of streamers
from pointy objects as that charge tries to meet the downward leader, also
see st elmo's fire... while these streamers are relatively low current they
are evidence of the build up of charge on the ground and all objects under
the leader. They can have enough charge to zap sensitive electronics like
tv preamps, satellite dishes, and other not so well protected pointy things.
Then suddenly the leader connects to a streamer and some of the charge is
neutralized very quickly... what happens to the rest of it??? Now it is
repelling from itself and can cause a rush of current away from the stroke
attachment point, especially if there are wires that were not hit that
provide a good path away from the stroke.
Lightning is indeed a very complex phenomenon, and obviously requires great
respect, especially when you are intentionally putting up a big pointy
object in your back yard!
David Robbins K1TTT
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: firstname.lastname@example.org [mailto:towertalk-
> email@example.com] On Behalf Of Jim Lux
> Sent: Thursday, January 27, 2005 22:03
> To: K8RI on Tower Talk; firstname.lastname@example.org; 'Tower Talk List'
> Subject: Re: [TowerTalk] Conductive Concrete and Grounding
> At 12:18 PM 1/27/2005, K8RI on Tower Talk wrote:
> >Trying to remember my theory... It's been a long time.
> >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.
> Lightning stroke currents (in a target that's being hit) are typically
> modeled (and equipment is tested) with a so-called double exponential
> y(t) = exp(-at)-exp(-bt)
> typical rise time(10% to 90%) is 2 microseconds, fall time to 50% is 50
> >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.
> Most lighting is 3-4 strokes. More at
> >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
> >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
> >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
> >but depends on the sending speed as well as the characters being sent.
> More that the bandwidth depends on the rise and fall time of the keying
> waveform, as well as the sending speed.
> >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
> >wave. In that case the power is basically a summation with some limit
> >the power drops off at a given rate with frequency.
> Even square waves have decreasing power for odd harmonics. A sawtooth
> (which is a better representation of a lightning stroke) has decreasing
> power for ALL harmonics. But, a harmonic representation is not a very
> one for lightning.
> You've essentially got a single shot impulse here, so you probably don't
> want to use a harmonic series representation.
> With the miracle of modern computation.. I built a waveform some 131072
> samples long, sampled at 1 nSec intervals, for a 1.5/50 microsecond
> impulse, then calculated the power spectrum. Most of the power is down
> quite low. By the time you get to even 1 MHz, you're already 60 dB down.
> It's important to NOT confuse the spectral characteristics of the actual
> stroke current with the spectral characteristics of signals that may be
> induced by an adjacent stroke, or with the spectrum of the field radiated
> by the stroke a long way away.
> The RF emissions (radiated) from the stroke does have a significant
> components well up into the tens of MHz. Partly because the "antenna"
> the lightning stroke itself) is bigger (in wavelength terms) for higher
> frequencies. Sure, the lightning stroke has a huge amount of power down
> 10kHz, but it's also a pretty poor antenna for that frequency. It's a
> positively giant antenna for 20MHz or 50 MHz. There are also some
> interesting spectral components coming from the stepped nature of the
> typical lightning stroke.
> If you make a simple approximation and put in a 20dB/decade correction,
> power vs frequency drops smoothly down to about -10dB at 1 MHz, sort of
> asymptotically converging to around -14dB from 3-4 MHz on out. This is
> relative to a +50dB for DC (the dominant component) The analysis had
> frequency bins about 7kHz wide.
> And, when it comes to induced currents, it gets even more complex. Then
> you have to take into account the frequency selectivity of the "victim
> 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.
> TowerTalk mailing list
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.
TowerTalk mailing list