FM radio interference from planes

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)

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

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 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..."

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.

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.

"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

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..."