>
> So is it possible that the nature of the arcing, flashover or corona is
> resonating within specific bands and/or at specific frequencies - or is that
> highly unlikely?
Greg,
I think "narrowband" sources that have lots of 60Hz and harmonics in them
might be things like light dimmers and switching power supplies that operate
at a higher carrier frequency.
Let's say you've got a switching supply that operates square-wave at 50kHz.
Let's say the manufacturer cheaped out and decided to not install any filter
capacitors after the rectifying section (or the filter capacitors failed?).
That would make the supply turn on and off every half cycle of 60Hz.
Of course, they didn't bother shielding it.
So you've got a 50kHz square wave generator (that has plenty of power up in
the high harmonics, like the 36th harmonic:
50kHz * 36 = 1.8MHz
Now, start turning that source on and off at 60Hz. That's *modulation*
If the turn on/off is hard edged (which it probably would be), then you're
modulating that 50kHz + harmonics source at 60Hz + harmonics.
So up there at 1.8MHz you might get something like :
(50kHz + 60Hz)*36 = 1.80216MHz
(50kHz + 120Hz)*36 = 1.80432MHz
You can also get stranger mixing products like the 18th harmonic of (50kHz
+60Hz) mixing with the 18th harmonic of (50kHz+120Hz) to give you something
around 1.803.
So I think generally speaking, you can get a big mess of modulation products
centered on harmonics of the "carrier frequency" that some badly designed
high frequency switching electronics uses, but when you listen on the radio
and demodulate that signal, what you hear in your passband is something that
sounds like it's 60Hz and harmonics, just like power line noise.
I'm going to cook up one more reply and actually calculate the spectrum of
this, because I think it might be interesting. Be back in a minute.
73
Dan
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