FM radio interference from planes

"Jim Macklin" wrote in message
news:CSVtg.68473$ZW3.64903@dukeread04...
What did I say that was wrong [other than the typo you
should have been your]. The comment about FCC rules...they
do frown on transmitting or retransmitting the wrong
signals.
*snip*
| Jim
|
|



He isn't transmitting anything on the wrong freq, he is receiving signals
that are originating from a freq outside the supposed range of the FM system
he bought.

That system he is using is undoubtedly a 'PART 15' device, with a label or
literature reading:

"This device complies with part 15 of the FCC Rules. Operation is subject
to the following two conditions: (1) This device may not cause harmful
interference, and (2) this device must accept any lnterference re-
ceived, including interference that may cause undesired operation."

So he can either live with it, find a HAM to build a filter, or take it back
and get something else.

"Jim Macklin" wrote in message
news:CSVtg.68473$ZW3.64903@dukeread04...
What did I say that was wrong [other than the typo you
should have been your]. The comment about FCC rules...they
do frown on transmitting or retransmitting the wrong
signals.

Forget it, Jim; the other Jim is exercising his depleted ego again.


"RST Engineering" wrote in message
...
| Macklin, I like you, you are a nice guy. But sometimes
you give advice that
| is way the hell out of your depth. Like this one.
|
|

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?

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?

"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)

On Fri, 14 Jul 2006 14:54:56 -0700, "RST Engineering"
wrote:


Consider the most probable cause. An FM receiver at 104.1 has a local
oscillator at 114.8 MHz to produce an IF of 10.7 MHz. Not only will that
beat with 104.1 on the low side of the LO, it will beat with 125.5 on the
high side. Most FM receivers have at least a +/- 50 kHz. wide IF strip to
allow the stereo subcarrier at 38 kHz. to come through, so 125.45, 125.5,
and 125.55 will come through as well.

The OP is in California bay area. Oakland Center in that area is
125.45. Southwest Airline flies to SJC, so it all makes sense...
Case solved.

-Nathan

OK, so we think we have a possible reason for the interference. Now, if the
OP will please describe for me what sort of antenna (s)he has on the
RECEIVER, I'll make a best guess as to how to mitigate the problem.

And no, in response to another post, if a device certified to part 15 and
still meeting part 15 standards interferes with another part 15 device,
neither interferer nor interferee is responsible for cleaning up any
interference. Or, in English, if my little transmitter is screwing up your
little receiver, I ain't gotta do nothing about it.

As for selling knock-off unlicensed transmitters, last I looked nobody in
their right minds is going to expose themselves to a $10K fine for each
device so sold. I defy anybody to go into a store anywhere in this country
and buy a little FM transmitter (they sell them in K-Mart and WallyWorld)
without a certification sticker on them and the FCC type cert number.

Jim



The OP is in California bay area. Oakland Center in that area is
125.45. Southwest Airline flies to SJC, so it all makes sense...
Case solved.

-Nathan

Consider the most probable cause. An FM receiver at 104.1 has a local
oscillator at 114.8 MHz to produce an IF of 10.7 MHz. Not only will
that
beat with 104.1 on the low side of the LO, it will beat with 125.5 on
the
high side. Most FM receivers have at least a +/- 50 kHz. wide IF strip
to
allow the stereo subcarrier at 38 kHz. to come through, so 125.45,
125.5,
and 125.55 will come through as well.
************************************************** ********************************************

Jim, you are preaching in Sanscrit... Unless they are ham radio ops, or
have a degree in EE, they won't even begin to comprehend..

0: this is what cheap radio equipment gets you
1: his receiver receives many, many radio frequencies, not just the one
it is tuned for
2: his 'transmitter' is radiating RF on more frequencies than 104.1
3: his receiver is actually radiating RF into the receiver antenna and
broadcasting
4. the combination of RF from the transmitter mixing with the many RF
signals present on the receiver's antenna terminals cause "phantom"
signals to be heard

To cure this he needs to put a filter on the transmitter output and one
on the receiver input... Or simply turn the FM transmitter off...

denny

"RST Engineering" wrote in message
...
Macklin, I like you, you are a nice guy. But sometimes you give advice
that is way the hell out of your depth. Like this one.

Jim, I don't know you except by what you post here, but you've gotta be the
most pompous PITA in the Western Hemisphere.





"Jim Macklin" wrote in message
news:AVTtg.67782$ZW3.26133@dukeread04...
104.1 FM is below the aircraft navigation and below the
communication assigned frequencies. But there are harmonic
signals that you cheap FM transmitter is not screening out.
Are you using a wired connection from the PC to the FM
transmitter or a WiFi or other radio? Most likely you're
getting the interference in that way.

You may be in violation of FCC rules.

Probably not, with a store-boughten transmitter. And harmonics (which are
integral multiples of a fundamental signal) probably have little to do
with it.

Consider the most probable cause. An FM receiver at 104.1 has a local
oscillator at 114.8 MHz to produce an IF of 10.7 MHz. Not only will that
beat with 104.1 on the low side of the LO, it will beat with 125.5 on the
high side. Most FM receivers have at least a +/- 50 kHz. wide IF strip to
allow the stereo subcarrier at 38 kHz. to come through, so 125.45, 125.5,
and 125.55 will come through as well.

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.

Having said all that, the real problem is to keep whatever small amount of
aircraft band com energy is present at the front end from getting into the
receiver.

And, before we start off on a wild goose chase, I'd advise the OP to get a
small handheld aircraft band transceiver or scanner and see if the real
transmitter is somewhere around 125.5 MHz.. It is always good to be able
to do a math calculation to see exactly what is getting into what before
spending a lot of time chasing your tail.

For those who say "you can't hear AM on an FM receiver", I say
horsefeathers. It may be reduced in volume, it may be distorted, but it
will get through.

Do the test, tell me what frequency from 118-136.975 MHz. the aircraft is
on, and we'll go from there.

Jim

grin


"Matt Barrow" wrote in message
...
|
| "RST Engineering" wrote in
message
| ...
| Macklin, I like you, you are a nice guy. But sometimes
you give advice
| that is way the hell out of your depth. Like this one.
|
| Jim, I don't know you except by what you post here, but
you've gotta be the
| most pompous PITA in the Western Hemisphere.
|
|
|
|
|
| "Jim Macklin"
wrote in message
| news:AVTtg.67782$ZW3.26133@dukeread04...
| 104.1 FM is below the aircraft navigation and below the
| communication assigned frequencies. But there are
harmonic
| signals that you cheap FM transmitter is not screening
out.
| Are you using a wired connection from the PC to the FM
| transmitter or a WiFi or other radio? Most likely
you're
| getting the interference in that way.
|
| You may be in violation of FCC rules.
|
| Probably not, with a store-boughten transmitter. And
harmonics (which are
| integral multiples of a fundamental signal) probably
have little to do
| with it.
|
| Consider the most probable cause. An FM receiver at
104.1 has a local
| oscillator at 114.8 MHz to produce an IF of 10.7 MHz.
Not only will that
| beat with 104.1 on the low side of the LO, it will beat
with 125.5 on the
| high side. Most FM receivers have at least a +/- 50
kHz. wide IF strip to
| allow the stereo subcarrier at 38 kHz. to come through,
so 125.45, 125.5,
| and 125.55 will come through as well.
|
| 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.
|
| Having said all that, the real problem is to keep
whatever small amount of
| aircraft band com energy is present at the front end
from getting into the
| receiver.
|
| And, before we start off on a wild goose chase, I'd
advise the OP to get a
| small handheld aircraft band transceiver or scanner and
see if the real
| transmitter is somewhere around 125.5 MHz.. It is
always good to be able
| to do a math calculation to see exactly what is getting
into what before
| spending a lot of time chasing your tail.
|
| For those who say "you can't hear AM on an FM receiver",
I say
| horsefeathers. It may be reduced in volume, it may be
distorted, but it
| will get through.
|
| Do the test, tell me what frequency from 118-136.975
MHz. the aircraft is
| on, and we'll go from there.
|
| Jim
|
|
|
|
|