FM radio interference from planes

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

"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

"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

"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.
|
|

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
|
|
|
|
|

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..."

"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

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..."

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..."

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)