FM radio interference from planes

"Larry Dighera" wrote in message
...
On Fri, 14 Jul 2006 14:54:56 -0700, "RST Engineering"
wrote in
::

The OP said that he doesn't get any interference when his transmitter is
off, so my best place to start looking is the front end of the receiver,
where the normal filtering of any decent FM receiver should take care of
the
"image" problem. However, when a very strong signal (like from a 100 mW
legal transmitter) comes blowing into the front end of the receiver from a
few feet away, crossmod and intermod are NOT your friend, and no designer
in
this world can make a brick wall filter that will take care of it.


So, you're saying, that although the aviation AM radio energy is not
being transmitted by the local MP3 FM transmitter, the receiver's
front (RF amplifier) is so overwhelmed by the local MP3 FM
transmitter's signal, that it enables aviation AM radio energy to
directly enter the receiver through cross-modulation and
inter-modulation? Did I understand you correctly?

I noted that as the most probable to me given the data presented. We could
postulate several wild and hairy schemes of harmonic mixing, but the most
likely suspect is inter or cross mod. The front end is not overwhelmed; it
is getting about the same amount of input that a commercial station in town
would present to the antenna, and the result SHOULD be the same. You can't
bias an amplifier to run ultralinear across a wide range of input voltages.



Isn't there also a possibility that nearby aviation AM radio energy is
entering the local MP3 FM transmitter through the power lines (or
transmitting antenna), and causing it to retransmit that aviation
content in addition to the MP3 content?


Not likely. Unless a very good case can be made, aviation ground stations
(tower, ground, atc) are limited to 25 watts carrier out. The ground
station would have to be literally in the OP's back yard to get into the
audio circuitry of the transmitter. As for "through the power lines", most
of these little transmitters are powered by wall warts that do a pretty fair
job of isolating line from equipment. One way of proving this to yourself
is to power the unit from a 9 volt battery and see if you get the same
result(s).

Have we established that the audio from the am source is not being
generated in the PC?

Is the OP using line-out or headphone audio to drive the Part-15
device? If the latter, where is the PC headphone volume set?

Don (Another oar to muddy the water)