On 7/31/2014 3:36 AM, Lostgallifreyan wrote:
Jerry Stuckle wrote in news:lrcavl$dok$2@dont-
email.me:

On 7/30/2014 5:08 PM, Lostgallifreyan wrote:
Jerry Stuckle wrote in news:lrbmkt$ohq$1@dont-
email.me:

Not necessarily. Even if there were physical vibrations, they may be
out of the audible range. But even vibrations are not common. You were
lucky you could hear it.


Well, it was like a flanger having a psychedilc fit. Hard to ignore!
Fortunately easy to cure. I imagine that many other sources of wild RF
oscillation might affect the sound like that.


Not really. To affect the sound, you need something which will respond
to the RF in a physical vibration manner (i.e. magnetic), and the signal
must be in the audio spectrum.

Neither is very likely.


To produce audio from RF directly all that is needed is for the difference (a
beat frequency) to fall within the audio pass band of the equipment. A wildly
varying frequency of oscillation could make a sweep of audio pitch. Timbre or
variability of effect might give plenty of clues as to cause. That's what led
me to find and fix the original oscillations in the dodgy ground plane
connection in the preamp casing. If it can happen this way on one (tiny)
scale, I see no general reason why it could not happen on the larger scale
with feedback between antennas a hundred feet apart. Whether I can use it
diagnostically is questionable, but I think it can exist. Given the number of
frequencies picked up by the antenna, I find it hard to beleive that NO stray
feedback oscillation would differ from any one of them by a value that puts
it in the audio band. I'm fairly sure I'd hear something, especially if
choosing a clean audio signal over which it woudl easily show up.


Yes, I know how it works - I've held both amateur and commercial
licenses for well over 40 years and worked on almost everything from $40
CB sets to multimillion dollar mainframe computers. I've even done some
design in my free time. I suggest you not try to teach those who know
more than you your "facts".

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.

Once you have this, you need something to generate a strong enough
magnetic field in the audio frequency range to act as a driver, and
something close enough and made of a magnetic material to vibrate.

I've seen a fair number of spurious emissions over the years (mostly
from VHF/UHF radios). Every one of them has been RF, and none of them
have created audio oscillations.

A few months ago we even had a case right here where a VHF radio in a
county bus had a spur on the input to the local 2 meter ham repeater.
Once again, no indication in the bus this was occurring. The county
found out about it only after the hams contacted them.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================