A true "DC" signal (i.e., theoretically pure DC) is constant *for all time*, from -infinity to +infinity. The spectrum (Fourier transform) of such a signal has a component at zero frequency only.
Any other signal (including an "almost pure DC" that is constant for long periods of time, short of infinite), will always have components at non-zero frequencies, due to the transition from zero current to the specified DC when the signal appears, and back to zero when the signal is removed. The amplitude of these non-zero frequency components is determined by the time since the transition (longer time since transition yields lower non-zero frequency amplitudes), the amplitude of the resulting current, and the slew rate (faster transitions yields higher non-zero frequency amplitudes).
Although we commonly speak of "DC" when discussing practical circuits and systems, we never encounter true DC, since the signal is never constant for infinite duration; however, a signal that is constant for long times (relative to the time periods we care about) can be approximated as DC for most engineering purposes. Lightning is NOT such a signal.
When he referred to the concept that lightning is DC, I believe that Jeff L. meant that the current in a lightning strike never changes
*direction*. However, it clearly changes amplitude during the time of interest, on the order of thousands of amperes in tens of milliseconds. Thus, the resulting spectrum will have significant components at non-zero values of frequency, as the various posted data indicate.The amplitude of the lightning signal will be non-zero for all values of frequency from -infinity to +infinity (as for any impulse-like current function). Thus, while it contains "RF" (i.e., signals in the RF spectrum), it will generally not have narrowband (CW) components at any particular frequency; thus, a narrowband filter (such as a 1/4 wave transmission line shunt) will not be effective to direct or contain lightning.
However, if the signal you are trying to *protect* is narrowband (e.g., a 2.4 GHz signal), a transmission line shunt can appear as a low impedance for all of the lightning components other than the desired 2.4 GHz. Thus, to the extent that the shunt is really a low impedance (good connection to earth, good conductivity, etc.) it *may* help somewhat. Of course, the filtering characteristics of a simple transmission line shunt are not spectacular; IIRC it appears as a single-pole filter, rolling off at only 6 dB/octave. Finally, if the lightning current melts the shunt conductor, all is again lost.
-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX
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