[Lowfer] 8820 Hz question - maybe OT
Clint Turner
turner at ussc.com
Tue Dec 2 16:59:15 EST 2014
There is no frequency above which an AC electromagnetic signal "starts"
to radiate - it is all RF to some extent - it just depends on the scale
of the conductor involved. Were we (humans) the size of ants, 80 meters
would be to us the sort of scale that LF/VLF is to us, now, and on a
planetary scale, LF/VLF wavelengths aren't incredibly long.
In order for a signal of a given frequency to be radiated from antenna
that is a tiny fraction of its wavelength - which would be the case for
"audio" frequency RF - there needs to be an efficient means to match
it. Practically speaking, this would involve either a wire (very high
voltage) or a loop (very high current) - either (practically!) being a
fraction of the wavelength in size. Neither would have the necessary
"aperture" to radiate effectively, and as a practical matter either one
will have tremendous losses: With the wire antenna it would
matching/ground losses, with the coil it would be matching and
resistive/skin-effect/matching dominantly.
Necessarily such a tiny antenna will have a "radiation resistance" of a
very tiny fraction of an ohm, so if the antenna and/or its
matching/ground losses were anywhere near an ohm (several ohms is much
more likely!) you can see that the losses will pile up as orders of
magnitude!
Fortunately, Shannon's theories dictate that given a low enough signal
rate with a narrow enough detection bandwidth, even such a ridiculously
small signal radiated by an installation such as the one assembled may
be detectable over long distances by people who are willing to go to
equally ridiculous lengths and spend absurd amounts of time in doing
so! (Like me!)
73,
Clint
KA7OEI
N8OOU wrote:
> Guys, I have been trying to remember my electronics and physics
> theory, and can't bring an answer to this question back to the top. I
> don't know if it is off topic for the list, if so I apologize, and
> welcome direct replies.
>
> My question is; what are the conditions or parameters for a signal
> like 8820 Hz to change from being contained in the magnetism of a
> transformer or speaker voice coil to being radiated from an antenna?
>
> I'm thinking I remember it being said that at some magic frequency
> above the audio range a signal starts to radiate. Obviously that
> conclusion is not true. I spent some time on google last night and
> didn't find an answer.
More information about the Lowfer
mailing list