FM radio interference from planes

[rb]

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.

[Jim Macklin]

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.
|

[Bob Gardner]

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.
>

[Peter Duniho]

"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

[Peter Duniho]

"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.

[RST Engineering]

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

[Larry Dighera]

On 14 Jul 2006 14:05:42 -0700, "rb" >
wrote in om>::

>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.

[Jim Macklin]

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.
|
|

[Jim Macklin]

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
|
|

[Larry Dighera]

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?

[Peter Duniho]

"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

[RST Engineering]

"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).

[Don Tuite]

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)

[Michael Ware]

"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.

[John[_2_]]

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).

[John[_2_]]

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.

[Jim Macklin]

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
|
|

[Nathan Young]

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

[Morgans[_3_]]

"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

[Denny]

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]

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
>
>
>
>

[Peter Duniho]

"John" > wrote in message ...
>> 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.

No, I don't.

> 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.

And as a certified Part 15 device, any inteference it may cause in another
Part 15 device (pretty much the only thing the neighbors are going to have)
is perfectly legal.

If he were running an illegal transmitter, that would be different. But a
legal transmitter can interfere with the neighbors until the cows come home,
and the FCC just doesn't care.

Pete

[Matt Barrow[_2_]]

"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
>
>
>

[Matt Barrow[_2_]]

"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.
> |
> |

[Jim Macklin]

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
| >
| >
| >
|
|

[Don Byrer]

On Fri, 14 Jul 2006 14:33:33 -0700, "Peter Duniho" <
>
>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).
>
Hmm...inquiring minds want to know...is the ATIS around 128.7-128.9
'ish???

--Don
Don Byrer
Radar Tech & Smilin' Commercial Pilot Guy
Amateur Radio KJ5KB
kj5kb-at-hotmail.com

"I have slipped the surly bonds of earth; now if I can just land without bending the gear..."
"Watch out for those doves...<smack-smack-smack-smack...>"

[Peter Duniho]

"Don Byrer" > wrote in message
...
>>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).
>>
> Hmm...inquiring minds want to know...is the ATIS around 128.7-128.9
> 'ish???

As a matter of fact: 128.65

I guess that's "around" 128.7 :)

I should mention that this particular car radio is highly susceptible to
radio interference generally. Down at the lower end of the "dial", say
93.3-ish and below, it's quite common for me to get interference from cell
phone towers. Or, at least it's my presumption that it's cell phone towers,
since they are pretty much the only ubiquitous kind of RF interference that
I know of.

When the interference happens, whatever I'm listening to turns to
high-volume static, without any intelligible signal at all. It practically
never occurs while I'm standing still, so presumably I'm moving into
interference range and then back out (the interference generally lasts
between 30 seconds and a few minutes, depending on how fast the car is
moving and whether I get stuck at a light :) ). And it never happens on the
higher frequencies (when I do get static, I can just switch to a
higher-frequency station and everything is fine there).

Pete

[Don Byrer]

On 14 Jul 2006 14:05:42 -0700, "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.


OK...here's my take on this...
I agree with RST Jim and John "Nospam" :)

Such mixing does occur...I maintain ground radios at CLE...Cleveland
OH. Our main transmitter site is on the main road next to the
airport. 10-watt radios on 60-ish foot towers....the 'slant range' to
the antenna could be ~150 feet if driving by. I have experienced
numerous instances of interference to FM broadcast stations as well as
2m (144-148 Mhz) amateur radio reception when driving by the site.
Several friends have also experienced this and queried me about it.

This could also easily happen with a nearby micro-power FM
transmitter and a much higher power 25-50W (?) airborne transmitter at
a few thousand feet away.

Looks like the FCC specs are 250 microvolts /meter measured at 3
meters. And I bet the transmitter in question is WAY less than
that, most are. I have used several FM transmitters meant for MP3
player use in a car and found them to be poor...and they only have to
go a few feet, even to a rear fender antenna.

On a similar note... I once had a complaint that our 2m( 146 MHZ)
Skywarn transmissions were interfering with Airband
communications...from someone using a scanner. He was sitting out in
front of our National Weather Service office (on the airport) at the
time, about 150 feet from the 2m antennas!

He was very concerned, but once we explained what had happened, AND
that I worked at the airport AND we hadn't had problems in several
years of operation...he relented...

--Don Byrer

Hey "rb"...you haven't posted since to say "thanks", ask further
questions, provide further info as requested or even to disagree.
How about it???



Don Byrer
Radar Tech & Smilin' Commercial Pilot Guy
Amateur Radio KJ5KB
kj5kb-at-hotmail.com

"I have slipped the surly bonds of earth; now if I can just land without bending the gear..."
"Watch out for those doves...<smack-smack-smack-smack...>"

[Don Byrer]

On Sat, 15 Jul 2006 22:37:55 -0700, "Peter Duniho"
> wrote:

>"Don Byrer" > wrote in message
...
>>>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).
>>>
>> Hmm...inquiring minds want to know...is the ATIS around 128.7-128.9
>> 'ish???

>
>As a matter of fact: 128.65
>
>I guess that's "around" 128.7 :)
>
Close enough...within the bandwidth anyway...think about how well you
can receive a strong FM station at "one channel off" may be fuzzy,
but understandable. And this being AM, it may come in clear quite a
few KHz off...even on an FM radio.

>I should mention that this particular car radio is highly susceptible to
>radio interference generally. Down at the lower end of the "dial", say
>93.3-ish and below, it's quite common for me to get interference from cell
>phone towers. Or, at least it's my presumption that it's cell phone towers,
>since they are pretty much the only ubiquitous kind of RF interference that
>I know of.
Could be all the other transmitters....VHF/UHF comms, paging, etc,
that may ALSO be on those towers...
>
>When the interference happens, whatever I'm listening to turns to
>high-volume static, without any intelligible signal at all. It practically
>never occurs while I'm standing still, so presumably I'm moving into
>interference range and then back out (the interference generally lasts
>between 30 seconds and a few minutes, depending on how fast the car is
>moving and whether I get stuck at a light :) ). And it never happens on the
>higher frequencies (when I do get static, I can just switch to a
>higher-frequency station and everything is fine there).

been dere, done dat...not the high/low freq difference, but it
happens.

--Don
Don Byrer
Radar Tech & Smilin' Commercial Pilot Guy
Amateur Radio KJ5KB
kj5kb-at-hotmail.com

"I have slipped the surly bonds of earth; now if I can just land without bending the gear..."
"Watch out for those doves...<smack-smack-smack-smack...>"

[Thomas Borchert]

Matt,

> > 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.
>

The person involved not-withstanding, what's more pompous, posting about
something you don't know much about or saying someone else did that? <gd&r>

--
Thomas Borchert (EDDH)

[Roger (K8RI)[_1_]]

On Fri, 14 Jul 2006 16:43:15 -0700, "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
>
>> 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).
>

If it is a part 15 device (very low power no license required. This
includes the wireless mikes, remote speakers, remote weather stations,
WiFi computer networks, and a whole bunch of other *stuff*) the
*required* sticker on the back says two things. He *may not* cause
any interference and has to accept any he receives.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

>Pete
>
Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

[Peter Duniho]

"Roger (K8RI)" > wrote in message
...
> If it is a part 15 device (very low power no license required. This
> includes the wireless mikes, remote speakers, remote weather stations,
> WiFi computer networks, and a whole bunch of other *stuff*) the
> *required* sticker on the back says two things. He *may not* cause
> any interference and has to accept any he receives.

What you (and the other responders) are failing to understand is that the
device in question is a TRANSMITTER. As such, it may very well interfere
with any other device, and such interference would be perfectly legal (as
long as the transmitter is operating within its legal requirements, a safe
assumption given the "off-the-shelf device intended for the use in which
he's applying it" qualification that started this subthread).

You cannot get a complete education on FCC regulations from a little sticker
found on the back of Part 15 devices.

Pete

[Roger (K8RI)[_1_]]

On Tue, 18 Jul 2006 14:19:48 -0700, "Peter Duniho"
> wrote:

>"Roger (K8RI)" > wrote in message
...
>> If it is a part 15 device (very low power no license required. This
>> includes the wireless mikes, remote speakers, remote weather stations,
>> WiFi computer networks, and a whole bunch of other *stuff*) the
>> *required* sticker on the back says two things. He *may not* cause
>> any interference and has to accept any he receives.
>
>What you (and the other responders) are failing to understand is that the
>device in question is a TRANSMITTER. As such, it may very well interfere
>with any other device, and such interference would be perfectly legal (as

No it is not. The FCC is very adamant about the interference
interpretation.

>long as the transmitter is operating within its legal requirements, a safe

That is the point! The "transmitting" device may not cause (is not
allowed to cause) interference even when it when it is operated fully
with in compliance to part 15. A number of companies are finding that
out the expensive way at present.

>assumption given the "off-the-shelf device intended for the use in which
>he's applying it" qualification that started this subthread).
>
>You cannot get a complete education on FCC regulations from a little sticker
>found on the back of Part 15 devices.

For part 15 devices that little sticker says it all.
The only thing you shouldn't have to worry about interfering with
would be another part 15 device.

As this one is operating in the FM broadcast band even if operated
within the limitations and his neighbors hear it and complain he'd
have to shut it down.

However the original post was not about the transmitter at all, but
him hearing aircraft radios through his system. That could be
intermod/front end overload, or images. As he's listening around 108
and aircraft are roughly 130 that gives a difference of 22 MHz which
shouldn't create an image problem unless one of the IFs runs in the 22
or 44 MHz range. So, I'd guess it's probably front end overload with
intermod.

I occasionally hear aircraft that would be transmitting on 126.45, or
118.45 on 147.00 MHz. However I'm directly under the GPS/RNAV 6 app
into Midland Barstow. Which means they are quite close to my antenna.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com
>
>Pete
>
Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

[Peter Duniho]

"Roger (K8RI)" > wrote in message
...
> No it is not. The FCC is very adamant about the interference
> interpretation.

Yes, it is. Just as the legal AM transmitter that's practically in my
backyard is not in violation of FCC rules when it bleeds into every
amplified electronic device in my house (and causes all sorts of other weird
issues too), so too is some other legal transmitter not in violation.

> That is the point! The "transmitting" device may not cause (is not
> allowed to cause) interference even when it when it is operated fully
> with in compliance to part 15. A number of companies are finding that
> out the expensive way at present.

Such as?

> For part 15 devices that little sticker says it all.
> The only thing you shouldn't have to worry about interfering with
> would be another part 15 device.

Again, you are missing the point. Practically any device that is in the
neighbor's house that might be interfered with is a Part 15 device (as I've
already stated in this thread). The neighbors won't have any valid
complaint, as their electronic devices are all required to accept
interference.

Pete

[RST Engineering]

> That is the point! The "transmitting" device may not cause (is not
> allowed to cause) interference even when it when it is operated fully
> with in compliance to part 15. A number of companies are finding that
> out the expensive way at present.

Roger, at one time I thought you had a grip on this radio and RF stuff. I'm
starting to believe not.


>
> However the original post was not about the transmitter at all, but
> him hearing aircraft radios through his system. That could be
> intermod/front end overload, or images. As he's listening around 108
> and aircraft are roughly 130 that gives a difference of 22 MHz which
> shouldn't create an image problem unless one of the IFs runs in the 22
> or 44 MHz range. So, I'd guess it's probably front end overload with
> intermod.

Come on, buddy. The image is at TWICE the IF from the intended. With the
standard 10.7 MHz. IF of an FM radio, the image is at 21.4 MHz higher than
the intended frequency, exactly where we predicted the image would be. If
he's listening at 107. the image will be at 128.4, right where it was found.

Jim