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In alt.engineering.electrical Eric wrote:
| wrote:
| In alt.engineering.electrical Eric wrote:
|
| | I can attest to vhf/uhf content in lightning strikes. I worked for a
| | communications outfit. We owned and maintained a number of comm sites
| | with towers and antennas. One strike on an antenna destroyed the LDF rf
| | cable all the way to the polyphaser at the bottom of the tower. It had
| | blowouts at about 1 foot intervals all down it's length suggesting a
| | 1/2 wave of about 1 foot or approx 460 mhz. That's one hell of a lot of
| | energy at that frequency..
|
| Apparently you had some kind of resonance involved. Maybe the antenna itself
| can cause that. Or the output tank circuit in the transmitter. Once you have
| the resonance to narrowband the energy, it would only take a reflection back
| up the line and you turn a propogating surge into standing waves.
|
| Pretty much what we determined. Also in another thread I stressed that
| the rise time by itself does not determine frequency content. One needs
| to know the rate of change, or slew rate, to determine that. A
| lightning pulse may have a rise time of 1.2 microseconds but in that
| short time the current can rise to thousands of amps, generating a large
| amount of vhf,uhf energy.

In some plots of voltage rises I've seen in the past, the rise was not at all
a smooth one. It went up in steps. Of course if one _thinks_ there will be
no VHF or UHF energy and does measurement with a ssytem only capable of lower
frequencies, that would smooth out how the rise appears.

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