FM radio interference from planes

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.

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.


"rb" wrote in message
ups.com...
|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.
|

What Jim said. In addition, aviation communication uses amplitude
modulation, not FM. You are getting some kind of rogue hetrodyning.

Bob Gardner

"rb" wrote in message
ups.com...
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.

"rb" wrote in message
ups.com...
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.

I can't speak for your specific situation. But generally, you've got a
couple of things going on: frequency harmonics mean that you can get signal
on different frequencies from that actually used by airplanes, and the fact
that airplane radios use AM mean that you don't need to even be on a
harmonic frequency (the AM radio station next to our house produces audible,
intelligible signals on practically any electronic device that has
amplification).

I discovered, once I started using an FM transmitter with my MP3 player in
the car, that once my FM transmitter is turned off, the car radio will pick
up the ATIS broadcast on the frequency I'm using for the FM transmitter
(107.3...but I doubt it matters much).

That said, I would be surprised if you are picking up transmissions from
airplanes themselves. I wouldn't expect their transmitters to be powerful
enough, nor close enough to you to produce a signal you can hear on your FM
receiver. It seems more likely to me that you are near a ground-based
station, with its relatively higher transmitting power, and are hearing that
side of the conversation.

The fact that it occurs only when your transmitter is actually turned on
suggests to me that the interference is actually in the transmitter itself,
especially if you see the same behavior in all of your receivers. Probably
there's some amplifier circuit somewhere in the transmitter that is picking
up the interference, which is then transmitted normally to each receiver.
Why this would override your intended signal in the transmitter, I don't
know...seems like it should be in addition to, rather than instead of (at
least that's been my experience with the interference from our neighbor's AM
radio station). But as I've also found with our various interference
issues, the exact manifestation of the interference can vary widely from
device to device (another reason that, if you are seeing the same behavior
in a variety of receivers, the interference is more likely affecting the
transmitter, since otherwise I wouldn't expect every receiver to be
interfered with in exactly the same way, assuming they aren't all of exactly
the same design).

We do have a couple of people who read this newsgroup who are very
experienced electronics engineers and who may be able to offer more specific
advice once they've read your post. However, you are not actually dealing
with an aviation issue here...you're actually asking about a radio
electronics issue, and as such you are likely to get better information from
a newsgroup dedicated to that topic, rather than aviation.

The one piece of information that you can get here that may be useful is the
range of aviation frequencies: they are from 118Mhz to 136Mhz (there are
navigation radios, on which some voice communications can occasionally
occur, from 108Mhz to 118Mhz, but I doubt these are what you're dealing
with), and as I mentioned before they use AM for the signal.

Hope that helps.

Pete

"Jim Macklin" wrote in message
news:AVTtg.67782$ZW3.26133@dukeread04...
[...]
You may be in violation of FCC rules.

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

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 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

On 14 Jul 2006 14:05:42 -0700, "rb"
wrote in . com::

now when I am broadcasting my stuff, I pick up planes instead.

Does this occur with other FM receivers, or just one?

How far away is the local airport from your location?

Is there a line-of-sight path to the control tower from your location?

Try putting a 0.1 mfd capacitor cross the power supply output
terminals of your FM transmitter. You can also try winding the power
cord around a ferrite doughnut.

If he is broadcasting and his power is above the proper
legal range, he MAY be in violation. He should be sure the
FM transmitter he has, has an FCC number and is not causing
any interference with the neighbors.


"Peter Duniho" wrote in
message ...
| "Jim Macklin" wrote
in message
| news:AVTtg.67782$ZW3.26133@dukeread04...
| [...]
| You may be in violation of FCC rules.
|
| Assuming he's using an off-the-shelf device intended for
the use in which
| he's applying it, that's extremely doubtful.
|
|

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.

BTW, back in the 1974 period I gave a student a night dual
x-c from Tulsa to OKC and back one night. We picked up
several TV stations on the old AM radios in the C150 we were
flying. So bad radios, that do not have properly working
filters and signal rejection can do strange things.

We also had fun with CB radios in our cars back in the late
60's, parking outside a church or at the drive-in movie and
being able to have our comments come over the PA system.
Poor shielding let our 5 W CB radio get into their amp.

Some times you give criticism that isn't helpful. In my old
age, I do make misteaks [mistakes] and often brake rules
[break] because I sometimes remember the wrong bit of
information. And sometimes I'm right [correct] by accident.
mine..."| "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[r] 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.
|


--
The people think the Constitution protects their rights;
But government sees it as an obstacle to be overcome.
some support
http://www.usdoj.gov/olc/secondamendment2.htm
See http://www.fija.org/ more about your rights and duties.


"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 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
|
|

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?