At 08:08 AM 12/1/2006, you wrote:
>Do Polyphaser lightning arrestors drain static? I thought they only shorted
>to ground after reaching a preset high voltage. He was only measuring 7
>volts maximum on his coax. If it did drain at 7 volts, what happens when
>you transmit with a KW into a less than perfect antenna? Does your RF also
>drain and not reach the antenna?
7 volts into what impedance. If it was the 10 Megs of a DMM, then
you're looking at a microamp at most (which is the sort of magnitude
of charging current one might expect). If you're looking at a
unterminated run of coax that's say, 100 ft long at 13 pF/ft, you're
basically charging a 1.3 nF capacitor with that microamp or so. Say
your switch breaks down at a couple kilovolts. 1 uA into 1.3nF gets
to 2.6 kV in 2 milliseconds.
Fortunately, there's not much energy (in a junction destroying sense)
in that system (1/2*C*V^2 = about 4 mJ with 2 kV at 1.3nF) Plenty of
energy to put a nice wideband RF spike out, though (and heck, it's
hooked up to antenna, so it will actually radiate!) In terms of
signal levels one might see...
If you discharge it, the pulse will last about 100-200 ns (the length
of the transmission line), so it will have a bandwidth of a few tens
of MHz. The instantaneous power is 4E-3/1E-7 or about 40 kW. RF
wise, that would be spread over, say, 40 MHz bandwidth (to make the
calculation easy), so the power spectral density is about 1 mW/Hz
(0dBm/Hz), which is a pretty good "pop" in your 3 kHz bandwidth...
I might be off by a few orders of magnitude here (4mJ at 500Hz rep
rate implies 2W, and that seems a bit high for rain charging.. but,
it's not out of the question for a fairly small Van deGraaff
generator, so maybe it really is that high). There's also resistive
losses in the cable, etc. (the current is quite high if it sparks
over, so IR losses suck up a lot of energy)
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