On Tue, 11 Jul 2006 12:13:49 -0700, Bill Turner wrote:
>As I understand it, lightning
>really does flow in one direction, making it DC but having a
>square-wave nature. Is that where the HF component comes from?
>Lightning doesn't really change directions, does it?
First, think of the current flow in the output stage of any
electronic amplifier. The instantaneous current flow has a DC bias
-- that is, it is always flowing in the same direction -- but it
has strong RF components, namely the signals they are amplifying.
The initial charge that sets off the lightning strike is the DC
bias, but the fact that it comes in the form of a fast risetime
pulse can cause oscillation in the resonant circuit that is the
discharge path.
As others have noted, the HF components comes from the very fast
rise time of the current discharge. Remember that from the
mathematical analysis that Fourier taught us more than a century
ago, any transient or non-sinusoidal waveform will resolve into an
infinite number of sine waves of increasing frequency (or if it is
an impulse, a series of sine waves of decaying strength). If the
single is repetitive, the higher frequency components will be
harmonically related to the repetition rate. The resulting
waveform can still come out with a DC bias, and the waveforms
don't necessarily all need to begin at the same instant (thus they
can resolve into any DC value, based on the physics of what else
is happening in the circuit).
Hope this helps.
Jim Brown K9YC
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