Jerry Stuckle wrote in news:lrdc2v$b15$1@dont-
email.me:

The facts a to have the beat frequency, you need two signals within
15kHz or so of each other. Those signals must be mixed, which means at
least one must be non-linear. This will give you a beat frequency in
the audio spectrum.


Ok, that nonlinear bit makes sense. If the signals do not do that then I'd
have nothing to detect the way I heard it. When I said audio I didn't mean
direct emission from something driven by strong RF. I was thinking it might
be heard from a weak signal added to the output audio signal, as happened
with that preamp's casing before I improved its connection to the PCB's
ground plane. The sound was a kind of whistling squeal, almost white-noise at
times. I imagined that feedbacking antennas might also produce this, and
that's the core of my question, whether or not this is true, or useful even
if it is.

By the way, I'm not trying to teach you anything. I know you know these
things or I'd not be here asking stuff. I was just stating what I know to try
to get to the bottom of this, and so you had some basis for pitching a reply
based on what you have reason to think I might understand.