FM radio interference from planes

"Jim Macklin" wrote in message
news:ASVtg.68472$ZW3.9262@dukeread04...
If he is broadcasting and his power is above the proper
legal range, he MAY be in violation.

Again, since it seems you didn't catch it the first time I wrote it:

Assuming he's using an off-the-shelf device intended for the use in which
he's applying it, that's extremely doubtful

He should be sure the
FM transmitter he has, has an FCC number and is not causing
any interference with the neighbors.

Whether he is causing any interference with the neighbors is irrelevant. As
long as it's approved by the FCC (and any off-the-shelf device intended for
the use in which he's applying it would be), the neighbors are required by
law to accept any interference (just as he and I and other people in similar
situations are required to accept interference from nearby AM transmitters).

Pete

"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)

"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.

Peter Duniho wrote:

"Jim Macklin" wrote in message
news:ASVtg.68472$ZW3.9262@dukeread04...
If he is broadcasting and his power is above the proper
legal range, he MAY be in violation.

Again, since it seems you didn't catch it the first time I wrote it:

Assuming he's using an off-the-shelf device intended for the use in which
he's applying it, that's extremely doubtful

Assuming that the device is properly constructed and adhering to standards that
is. Typically manufacturers can "self certify." YMMV.



He should be sure the
FM transmitter he has, has an FCC number and is not causing
any interference with the neighbors.

Whether he is causing any interference with the neighbors is irrelevant. As
long as it's approved by the FCC (and any off-the-shelf device intended for
the use in which he's applying it would be), the neighbors are required by
law to accept any interference (just as he and I and other people in similar
situations are required to accept interference from nearby AM transmitters).

You have that backwards. A part 15 device must not cause interference to
others. Conversely the user of the part 15 device must accept interference from
other users of the band. A local wireless (unlicensed) transmitter would
typically be a part 15 device.

This is assuming that the FCC has jurisdiction (i.e. USA).

rb wrote:

I bought a FM wireless transmitter to rebroadcast stuff from my PC
around the house. The only frequency I could find was 104.1 that was
clear without a station. But all of a sudden, now when I am
broadcasting my stuff, I pick up planes instead. How can this be? I
know it is from planes, as I hear words like "Southwest", etc. Without
my transmitter on, all I hear is static on 104.1
but when I turn on my transmitter, the planes seem to over ride my
signal.

Your transmitter may have an intermediate beat frequency of about 21.7
Mhz. This would explain the aircraft interference. 104.1 + 21.7 =
125.7. The aircraft you hear are most likely broadcasting somewhere
around 125.7 Mhz (which is in the middle of the air band), 125.7, 125.75,
126 Mhz, etc. Filtering out such images can be difficult. Your
electronics are unfortunately probably not very good at filtering this
out. Also, aircraft radios use AM, and your FM equipment should have a
filter to effectively remove AM signals, but apparently it is not doing
this well.

Are you fairly close to an airport that Southwest lands at? Your house is
probably under a sector that uses a frequency that your equipment is
sensitive to. It could be high altitude communications, although if you
are near an airport the aircraft would be closer and have a stronger
signal for you so I think that is more likely. (Aircraft radios only
transmit with a few watts, compared to maybe 50,000 watts for an FM
broadcast station. You are probably receiving broadcasts from the air,
unless you are very close to a ground transmitter station.

I have experienced this myself with just a cheap FM radio. Aircraft
frequency 118.5, radio station 96.9. Aircraft communications were not
very clear, but it was obvious what it was when planes were very close.

The sender/generator of interference is required to take
steps to mitigate such and the others are expected to adjust
their antennas and equipment as needed. But that
presupposes that the transmitter is working with Part 15 and
is not a knock-off or non-certified device. Also, I'm not
trying to answer the question, I said he MIGHT be in
violation. I have not read Part 15 in the last few months
and I am not a ham or radio technician.

My original answer/question was about using a wire or radio
to get the desired signal from his PC to the FM transmitter.
If it is a wire and it is not properly shielded with good
grounds at both ends, it could be acting as an antenna input
to the FM. A wire will bring more signal, with less loss
than a WiFi, which should be well shielded.


"Peter Duniho" wrote in
message ...
| "Jim Macklin" wrote
in message
| news:ASVtg.68472$ZW3.9262@dukeread04...
| If he is broadcasting and his power is above the proper
| legal range, he MAY be in violation.
|
| Again, since it seems you didn't catch it the first time I
wrote it:
|
| Assuming he's using an off-the-shelf device intended for
the use in which
| he's applying it, that's extremely doubtful
|
| He should be sure the
| FM transmitter he has, has an FCC number and is not
causing
| any interference with the neighbors.
|
| Whether he is causing any interference with the neighbors
is irrelevant. As
| long as it's approved by the FCC (and any off-the-shelf
device intended for
| the use in which he's applying it would be), the neighbors
are required by
| law to accept any interference (just as he and I and other
people in similar
| situations are required to accept interference from nearby
AM transmitters).
|
| Pete
|
|

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

"John" wrote

I have experienced this myself with just a cheap FM radio. Aircraft
frequency 118.5, radio station 96.9. Aircraft communications were not
very clear, but it was obvious what it was when planes were very close.

At OSH, I found that you could pick up Chicago center on the low range of an
FM radio, if you had the antenna just right, held your tounge in the correct
corner of you mouth, ect, ect. It was coming from a transmitter antenna on
the grounds, I suspect, and all that could be heard was the center side of
the transmission.
--
Jim in NC

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