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Interference in FM radio reception.

M

micky

Jan 1, 1970
0
AFAIK, nothing around here has changed, but in the last 3 months I
have been getting intermittent but strong interference in my FM radio
reception, and I'd like to find the source and stop it. .

I'm usually in bed when I notice this, and today for the first time, I
was able to get to two other radios and I saw the same interference
was on both of them for the same frequency, but it wasn't on the
Intenet version of same station.

It can last from 10 seconds to over an hour. It can be continuous or
go off and on occasionally, with off-periods that also vary in
length.. It sounds sort of like a fog horn, but a somewhat higher
pitch. Or the horn on a diesel locamotive. Or a steady note on a
trumpet. Except it often doesn't end as suddenly as they do, but
might have little noises for a second or two at the end. (I can't
remember how to describe the sound at the end.)

The sound can be medium or loud. That is, sometimes I can sort of
hear the radio program, usually talk, in addition to the noise. Other
times the noise overwhelms the program and I have no idea what they
are saying..

It affects 90.1 and 88.5 Mhz, either one and sometimes both. Both of
these stations normally come in perfectly. I live in Baltimore, and
these are DC stations, WAMU and WCSP, which is C-Span radio, (which is
broadcast only from DC). I haven't found it on 88.1 and afaicr
itdoesn't show up on frequencies much higher than 90.1.

It happens in the middle of the night some times, or today at noon, or
it seems any time.

My nearest neighbor said she wasn't home during one episode, but I
have otther townhouse neigbhbors farther away.

Any suggestions what the source of this might be?

Thanks for any help.
 
P

Phil Allison

Jan 1, 1970
0
"micky"
AFAIK, nothing around here has changed, but in the last 3 months I
have been getting intermittent but strong interference in my FM radio
reception, and I'd like to find the source and stop it. .

I'm usually in bed when I notice this, and today for the first time, I
was able to get to two other radios and I saw the same interference
was on both of them for the same frequency, but it wasn't on the
Intenet version of same station.

It can last from 10 seconds to over an hour. It can be continuous or
go off and on occasionally, with off-periods that also vary in
length.. It sounds sort of like a fog horn, but a somewhat higher
pitch. Or the horn on a diesel locamotive. Or a steady note on a
trumpet. Except it often doesn't end as suddenly as they do, but
might have little noises for a second or two at the end. (I can't
remember how to describe the sound at the end.)

The sound can be medium or loud. That is, sometimes I can sort of
hear the radio program, usually talk, in addition to the noise. Other
times the noise overwhelms the program and I have no idea what they
are saying..

It affects 90.1 and 88.5 Mhz, either one and sometimes both. Both of
these stations normally come in perfectly. I live in Baltimore, and
these are DC stations, WAMU and WCSP, which is C-Span radio, (which is
broadcast only from DC). I haven't found it on 88.1 and afaicr
itdoesn't show up on frequencies much higher than 90.1.

It happens in the middle of the night some times, or today at noon, or
it seems any time.

My nearest neighbor said she wasn't home during one episode, but I
have otther townhouse neigbhbors farther away.

** You will need to get a portable FM radio and walk about with it to find
where the source is.

Anecdotes:

1. Once had spike noise interference visible on the scope on my workbench -
it began in the early evening and was at a very steady rate of once in 6
seconds. On a portable AM radio, the clicking noise was very clear. I
tracked it down to a small, flashing, red neon sign in the window of a
pharmacy 60 metres away.

2. Had interference on TV at about the same time, visible only on VHF
channel 2 ( 63 to 70 MHz) - the colour would drop out regularly ( every 5 -
10 seconds ) and noise bands appeared. The problem was only there during the
day on work days.

Turned out to be a RF plastic welding machine on 27.12 MHz in a factory 200
metres away. The particular TV was known to be vulnerable to CB radio when
tuned to channel 2.

3. High frequency RF suddenly appeared on my scope one day while doing
testing at full sensitivity ( 5mV /div). It disappeared after 15 minutes but
then returned the next day. Turing up the time base speed showed it to be a
steady signal on 29 to 30MHz.

Switched on my radio scanner and soon found that my upstairs neighbours had
bought themselves a cordless telephone transmitting both ends on the
conversation on 30.15 MHz.


.... Phil
 
B

buddy

Jan 1, 1970
0
Cell towers around here Akron ohio rent tower space to different radio or
RF links they put their dish or arrays on the towers that have been alot of
RFI problems just thinking that maybe its the cause of the interference
problems
 
M

micky

Jan 1, 1970
0
AFAIK, nothing around here has changed, but in the last 3 months I
have been getting intermittent but strong interference in my FM radio
reception, and I'd like to find the source and stop it. .


First I'm going to find my MP3 player, which has an FM radio, and try
to circle in on the source of the noise. I didn't think the problem
could be so far away, so it's good that I'm looking farther now. .

There are two big ham radio clubs around here, and smaller ones in
adjacent counties. I go to their hamfests all the time. A few of
them even know me. The problem now is that I haven't kept a log of
when the interference occurs, so even if they're willing, I can't tell
them when to come over. I'll keep the log, and I'll call them
anyhow. Maybe one who likes foxhunting lives 5 minutes away, etc.

I plan to get back to you with more details when I know them, but it
may take a while.

BTW, the two stations I mentioned are HD, so Jeff, maybe that's why
they get the noise. So is 88.1, which is very close to 88.5 of
course, but 88.1 is a local station, not 45 miles away, so maybe its
signal is too strong for this interference to interfere????


Thaks for all the good advice.

P.S. There are also one or two buses that park 100 to 150 yards
from my house. Maybe they have some fancy new electronics. They have
an erratic schedule themselves, in that I thought there was never more
than one and it only parked unitl i's next route began. Maybe 10
minutes, but I've been paying attention and it's more complicated.
Al;so I live in the suburbs and didn't think they ran all night, but
next time the noise starts in the night I'll force myself to the
window to look. (Obviously I'm awake, but I don't like sitting up.)
 
N

N_Cook

Jan 1, 1970
0
Jeff Liebermann said:
Good description but not enough to identify the culprit. It doesn't
sound like a heterodyne and there's no indication of any additional
voice or music modulation making intermod an unlikely cause. If you
could record an MP3 audio clip and post it somewhere, it would be a
big help.

FM noise is fairly uncommon. FM was originally designed to ignore
amplitude modulated noise, which it does quite well. However, with
the introduction of HDFM, the digital modulation scheme included AM
components. The result is the sensitivity to AM interference has
increased. The noise description does not sound familiar so this is
unlikely. However, I'm curious if the unspecified maker and model
receiver is and HDFM receiver and were you possibly listening to HD1
or HD2.

Also, try a different FM receiver. If both radios receive the same
interference, then its probably being radiated over the air or
conducted on the power lines. However, if only one radio hears the
interference on the same channel, then it's possible that there's
something wrong with the receiver or the noise is being generated in
the radio.

Also, your description sound vaguely like RFI from a plasma TV. The
level, modulation characteristics, and frequency will vary with what
is on the screen. The timing pattern you describe sounds roughly like
a TV being turned on and off at random times. Do the times when it's
on coincide with prime time TV?

--
Jeff Liebermann [email protected]
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558



I've no idea if it would work but can you make a portable FM receiver
directional by placing in a metal tube? Perhaps with a metal plate under a
shoe with a grounding wire. Would have to be speaker output , so an earpiece
did not fed RF down
 
AFAIK, nothing around here has changed, but in the last 3 months I
have been getting intermittent but strong interference in my FM radio
reception, and I'd like to find the source and stop it. .

I'm usually in bed when I notice this, and today for the first time, I
was able to get to two other radios and I saw the same interference
was on both of them for the same frequency, but it wasn't on the
Intenet version of same station.

It can last from 10 seconds to over an hour. It can be continuous or
go off and on occasionally, with off-periods that also vary in
length.. It sounds sort of like a fog horn, but a somewhat higher
pitch. Or the horn on a diesel locamotive. Or a steady note on a
trumpet. Except it often doesn't end as suddenly as they do, but
might have little noises for a second or two at the end. (I can't
remember how to describe the sound at the end.)

The sound can be medium or loud. That is, sometimes I can sort of
hear the radio program, usually talk, in addition to the noise. Other
times the noise overwhelms the program and I have no idea what they
are saying..

It affects 90.1 and 88.5 Mhz, either one and sometimes both. Both of
these stations normally come in perfectly. I live in Baltimore, and
these are DC stations, WAMU and WCSP, which is C-Span radio, (which is
broadcast only from DC). I haven't found it on 88.1 and afaicr
itdoesn't show up on frequencies much higher than 90.1.

It happens in the middle of the night some times, or today at noon, or
it seems any time.

My nearest neighbor said she wasn't home during one episode, but I
have otther townhouse neigbhbors farther away.

Any suggestions what the source of this might be?

Thanks for any help.
One thing that strikes me about this is the random (erratic?) nature
of this interference, varying both in time and duration. To me, that
rules out many devices - common home appliances, factory equipment,
radio transmitters, etc. One thing that does operate on a similar
pattern is a telephone. Is it possibly a cordless telephone operating
on an oddball frequency?

PlainBill
 
M

micky

Jan 1, 1970
0
Think about something run by a thermostat, low control switch, or
motion detector. They tend to cycle erratically. Things like a
furnace,

The furnace would correspond to the end of the night, and the
beginning of the morning, which are common times for this. . At the
end of the night, it's cold for the night thermostat setting, and at
the beginning of the morning, it has to get hotter yet.

In some ways my neighbors are very friendly. Most will answer the
door at 9PM. OTOH, when I asked a couple to see how their new furnace
was installed, they looked at me strange.

I wouldn't have nerve enough, as someone suggested, to ask one to turn
off all his power, certainly not unless I had narrowed it down to his
house.
electric heater, water pressure boost pump, pool heater,
demand water heater, sump pump, etc.

Definitely does this even when there hasn't been any rain. I'm
literally the lowest house in the n'hood**, and would have heard my
sump pump when I was hearing the basement radio make the noise.

**And the way it works out, it's 200 yards or more to any house
outside the n'hood, all of which are also higher than I am. .
There are also some uncommon
devices that cycle. Negative ion generator, parking lot light
controller, external HDD or NAS boxes, motion detector operated
devices, iPhone background sync, etc.

Dang, I only rejected one from your whole list.
However, I've been fooled before with this pattern. I was looking for
such things, only to discover that the interference was coming from a
microwave oven at a local eatery. During lunch, the microwave ovens
were in use almost constantly. At other times, a minute or so at a
time. Kinda sounds like the original description.

Maybe I should go look for lights on when it does this in the middle
of the night. That's one thing I might do.
Dumb story:

There are no dumb questions or dumb stories. Only dumb people.

:~) Oops, I shouldn't bite the hand that feeds me. :~)
 
M

micky

Jan 1, 1970
0
The hams can be very helpful if properly approached. Much depends on
the available time, talent, experience, and hardware.


I can't tell from here. The problem is that the occupied bandwidth of
an FM station with HD1 and HD2 sub-carriers is more than the assigned
bandwidth:
<http://www.ham-radio.com/k6sti/hdrsn.htm>
Just read the first two sections, as the rest is all about fixing the
resultant noise.

Reminder: Please make an MP3 sound clip of the noises.

That will take me some time. I first have to find my Sanza Chip, or
Clip,or whatever it's called. It does record iirc. I've been sort
of looking for it for months, but now I have a reason to look harder.

The radio next to the computer has reception problems with these
stations, so the noise is muffled and only recognizeable because I
heard it on the other radios. Tonight I tried to fix up two AM/FM
clock radios, but only got the AM working on each.

I also have a 40 year old cassette recorder that probably works fine,
unless it doesn't I guess I coudl use that and then play it for the
computer mike. (I have a great computer mike. )
<http://www.dxzone.com/catalog/Internet_and_Radio/Sounds/>

Identifying Sources of Radio Frequency Interference Around the Home
"http://randombio.com/interference.html"

I'll check these all out.
Hint: Assumption is the mother of all screwups. Test your theories
and guesswork, even if you think it's improbable. I've been wrong and
surprised more times than I can comfortably admit. That's one reason
my domain is called LearnByDestroying.

LOL. Okay. I'll do my best.
 
Think about something run by a thermostat, low control switch, or
motion detector. They tend to cycle erratically. Things like a
furnace, electric heater, water pressure boost pump, pool heater,
demand water heater, sump pump, etc. There are also some uncommon
devices that cycle. Negative ion generator, parking lot light
controller, external HDD or NAS boxes, motion detector operated
devices, iPhone background sync, etc.

However, I've been fooled before with this pattern. I was looking for
such things, only to discover that the interference was coming from a
microwave oven at a local eatery. During lunch, the microwave ovens
were in use almost constantly. At other times, a minute or so at a
time. Kinda sounds like the original description.

Dumb story: Many years ago, I had a very difficult time trying to
find the source of short bursts of RFI. I tried for days and failed.
Oddly, the interference seemed wide spread and appeared almost
everywhere that I was sniffing. Eventually, the gears that drive my
brain engaged and I discovered that the Motorola Bravo pager that I
was carrying at the time, was the source. Argh.

Let's look at your list:
Think about something run by a thermostat, low control switch, or
motion detector. They tend to cycle erratically.

A thermostat - refrigerator, freezer, or heating / cooling tends to
cycle at a fairly constant rate, at least for the majority of the time
(an exception would be when a large amount of food is added to a
refrigerator, or a freezer, or when the set temerature of a thermostat
is changed). A motion detector would operate at erratic times, but
not for only 10 seconds.
Things like a
furnace, electric heater, water pressure boost pump, pool heater,
demand water heater, sump pump, etc.

Again, these would tend to operate on a regular cycle except for an
'on demand' water heater. However, the 'on demand' heater fails the
'on for one hour ' parameter.
There are also some uncommon
devices that cycle. Negative ion generator, parking lot light
controller, external HDD or NAS boxes, motion detector operated
devices, iPhone background sync, etc.
Again, these devices fail either the '10 second' or 'one hour'
parameter.

PlainBill
 
M

micky

Jan 1, 1970
0
There are motion detector lighting systems that work this way. For
example, a local eco-friendly business saves electricity by not having
the parking lights on unless it first detects motion. The lights are
timed to stay on for approximately twice the time it takes to walk
across the parking lot. I think that's about 4 minutes.

I have to say, this sounds like the opening scene from a murder
mystery. Only cars are big enough to start the lights. The woman
parks the car, away from the entrance so other cars won't ding hers.
She walks to the store, is almost there, remembers sometihing in the
car and has to go back, and on her second trip to the store, the
parking lot lights go off, and she hears footsteps behind her. We
only see her legs and his legs, and soon they are both running.

In the next scene, there are a lot of police standing around.
 
M

micky

Jan 1, 1970
0
Nope. They respond to people.

Dramatic license.
I park there all the time. As I
vaguely recall, the motion sensors are located about half way up the
light poles. They take a few seconds to come on to full brightness,
and likewise, fade slowly when they turn off. Since the parking lot
lights are independently controlled, it's possible to have them come
on in sequence as one walks slowly across the lot.


Considering the time of day and the neighborhood, that's quite likely.
Fortunately, having the lights come on when the prospective car thief
enters the parking lot tends to provide a rather strong deterrent.

If you want to sell this script, we're going to have to change some
things. If you won't cooperate, I'm not going to let you know when
the meeting is.
Possibly. The city police station is about 200 meters away.

The problem with finding sources of interference and such is the
hardware required. I used to do quite a bit of wi-fi sniffing,
searching for various sources of interference, leeches, hackers, DoS
sources, over-powered radios, and such. Same with searching for stuck
transmitters on commercial frequencies, foreign fishermen on US
frequencies, unlicensed operators, and premature LPFM stations. While
the equipment varies, it always seems to attract the attention of the
authorities. Walking through a busy shopping center parking lot, with
a fiberglass pole, topped with a small dish antenna, dragging a pile
of black boxes, with my face glued to a laptop. To the average
shopper, I was a cross between a terrorist and a visiting
intergalactic alien.

I'll bet!
No melodrama, but plenty of answering really
dumb questions. After a while, I learned to think first, and search
after. Have fun finding the interference source.

Thanks. I'll let you know how it goes. Though I've lost some
interest in finding the interfernce, and I'm turning my attention to
finishing the script and shopping it around.
 
J

josephkk

Jan 1, 1970
0
Argh. That's all wrong. I somehow merged the 21.4IF feedthru problem
with the CB harmonic filter. The lo-pass filter should be on the CB
xmitter, to reduce the 2nd harmonic that trashed Channel 2. The 54MHz
high pass filter goes on TV, to keep the CB radio from generating
harmonics in the tuner section.

Remind me not to post anything before my morning coffee fix.

Y0u had me worried there.

?-)
 
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