Interference in FM radio reception.

Thu Jan 05, 2012 11:59 pm   

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.

Fri Jan 06, 2012 12:25 am   

micky wrote:

The Russians have resurrected a souped-up version of The Russian Woodpecker
(Wikipedia that)! (AKA "Steelyard")

TRW was sited very close (~10km) to the nuclear power station at Pripyat,
near Chernobyl.

Now the whole area is overgrown with trees, but the antenna array still
stands.

And interestingly, as a westerner, I had assumed it was way north of where
I am, but actually, Pripyat/Steelyard are on almost exactly the same latitude
as my hometown in Kent (south-east England)!

I heard TRW on the radio when I was a kid. There was a documentary on TV
about it at the time.

As for your FM problem, sorry, to be honest, I don't know. But if you find out,
let us know here; I'd be interested to read what it was.


Martin

Fri Jan 06, 2012 12:35 am   

In article <86rbg7to7rinepav49pih0hefccqo905kk_at_4ax.com>,
micky <NONONOmisc07_at_bigfoot.com> wrote:


What you are describing, sounds somewhat to me as if it may be what
amateur-radio operators refer to as "doubling" - the result of two
signals of somewhat similar strength, on the same or near-by FM
frequencies at the same time. On ham 2-meter transmissions it often
has a characteristic "growling" sound. If one of the two signals is
significantly stronger than the other, you may hear the music or voice
from that signal, but in many cases neither transmission can be heard
clearly.


If I had to guess, I'd guess that there's somebody in your
neighborhood operating some sort of local-area FM transmitter. This
might be:

- A "broadcast your CD player music throughout the house" device, or

- a "feed music from your CD player or iPod, through FM, into your
car radio which doesn't have a direct input" device, or

- a full-fledged "pirate radio" station, or

- a legitimately-licensed "micropower" FM station (there are a few
although it's not at all easy to get a license), or

- some sort of malfunctioning device, in or near your house, which is
oscillating out of control and generating spurious frequency
carriers. Fluorescent light ballasts, electronic heaters,
defective house wiring, and computer-network gear can all do this.

In all of these cases (except possibly the licensed micropower
station) the offending transmission ought to be shut down, as (1) it's
interfering, (2) on a frequency it shouldn't be on, (3) without a
license.

It's also possible that your radios are being interfered with, by a
perfectly legal (but high-powered) transmission from a nearby
source... police or fire radio transmitter, ham radio, etc. Cases
like this are usually the result of something called "fundamental
overload" or "strong signal overload", and they are usually *not* the
legal fault or responsibility of whoever is transmitting. They are
(the FCC declares) the result of poor design of the receiving radio
(e.g. yours) - it's not adequately shielded or filtered - and it'd be
your responsibility to fix your radio. However, from the fact that
you're hearing the problem only on certain frequencies, while others
higher on the band are not affected, I don't think that this is the
case in your situation.

The fact that you're hearing it on multiple radios, gives you a pretty
good assurance that it's not just a single malfunctioning radio. Are
all of these radios powered from the AC mains, or are all of them
battery radios, or have you heard it on some of each type?

As to locating the source... well, you're probably in for an exercise
in radio direction finding! There are quite a variety of ways to
trace down the location of a transmission... some require lots of
equipment, some require as little as a small battery-powered portable
radio and a Pringles can covered in aluminum foil (the "body fade")
method.

A practical suggestion: see if you can locate one or more amateur
radio clubs in your area. Within the ham community, there are a fair
number of hams who enjoy "foxhunting" (radio-direction-finding
contests, looking for hidden transmitters), and others who have a very
specific interest in hunting for RF interferences sources and for
illegal transmitters (some of the ARRL "official observer" volunteers
have teams who do this). You may be able to locate some folks who
would be willing to come out, with some of their direction-finding
equipment, and help you locate the source of the interfering
transmission.

--
Dave Platt <dplatt_at_radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Fri Jan 06, 2012 1:03 am   

"micky"

** 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

Fri Jan 06, 2012 2:41 am   

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



"Phil Allison" <phil_a_at_tpg.com.au> wrote in message
news:9mms2rFgcbU1_at_mid.individual.net...

Fri Jan 06, 2012 7:26 am   

On Thu, 05 Jan 2012 17:59:54 -0500, micky <NONONOmisc07_at_bigfoot.com>
wrote:


(...)

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 jeffl_at_cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Fri Jan 06, 2012 4:49 pm   

On Jan 5, 4:59 pm, micky <NONONOmis...@bigfoot.com> wrote:

Portable FM radio, listen for the original noise, then walk around the
neighborhood.

Sat Jan 07, 2012 7:19 am   

On Thu, 05 Jan 2012 17:59:54 -0500, micky <NONONOmisc07_at_bigfoot.com>
wrote:



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.)

Sat Jan 07, 2012 9:50 am   

Jeff Liebermann <jeffl_at_cruzio.com> wrote in message
news:8e4dg79eff2ncffdhplvf3ojd06cua7doj_at_4ax.com...



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

Sat Jan 07, 2012 6:13 pm   

On Sat, 7 Jan 2012 08:50:51 -0000, "N_Cook" <diverse_at_tcp.co.uk> wrote:


Not really. The tube would need to be about 1/2 wavelength long
(about 1.5 meters) and would totally block the signal. If you run a
1/4 wave lengthwise slit down the length of the tube, you would be
able to hear something (at reduced sensitivity). However, it wouldn't
be very directional. I've built rotating tube type direction finders
using this principle, but at 2.4Ghz, where the size of the antenna is
more practical.


You can stop the earpiece from becoming an antenna with a simple
ferrite bead at the EP jack.

Much of what you suggest has already been done by hams for transmitter
hunts.
<http://www.homingin.com>
There's quite a bit of existing equipment available that will work.
Something cheap and crude are the various "homer" type direction
finders. Also known as TDOA (time difference of arrival).
<http://www.handi-finder.com>
<http://www.arrl.org/direction-finding>
<http://www.dxzone.com/catalog/Operating_Modes/Radio_Direction_Finding/>
etc..

AN/SRD-21 homer type direction finder manual (4.5MB)
<http://802.11junk.com/jeffl/AN-SRD-21/ANSRD-21%20manual.pdf>

Much of the technology use for the 2meter (145Mhz) ham band is
directly applicable to the 88-108 band if scaled. Unfortunately, all
the problems and headaches are also applicable (interference from
other stations, reflections, insufficient directionality, antenna side
lobes, etc).

Direction finding on the FM broadcast band is problematic because of
the potential for interference from adjacent channel stations. The FM
receiver IF bandwidth usually slops into the adjacent channels. In
the US, we have 200KHz channels, with allocations every alternate
channel. This reduces the problem, but doesn't eliminate it. In EU,
it's on 100KHz channels, possibly with alternate channel allocations.

However, methinks all this is overkill for finding a noise source. The
first thing I would do is turn off the power to the house, except for
the FM receiver. Many noise sources are conducted (through the power
lines) rather than radiated (over the air). If the noise goes away,
it's coming from something in the house. From there, it's just a
matter of turning off breakers until the location is isolated. Then,
turn off individual pieces of equipment. I've found an amazing
assortment of noise sources this way. If killing the power doesn't
find it, ask the neighbors to do the same (while carrying a portable
FM receiver). You'll be amazed at how many consumer devices generate
disgusting levels of RFI/EMI. From the description, the 10 sec to 1
hr duration is an important clue. That points to a thermostat, air
flow, or water demand operated device. It might also be an electric
heater. Killing the house power should be able to isolate it.

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

Sat Jan 07, 2012 10:39 pm   

On Thu, 05 Jan 2012 17:59:54 -0500, micky <NONONOmisc07_at_bigfoot.com>
wrote:


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

Sat Jan 07, 2012 11:51 pm   

On Sat, 07 Jan 2012 14:39:02 -0700, PlainBill_at_yawhoo.com wrote:


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.

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

Sun Jan 08, 2012 1:23 am   

Jeff Liebermann <jeffl_at_cruzio.com> wrote:


You can use an fm radio with a full length antenna. It will register in two
directions used horizontally. Some broad band harmonics are best tracked
down going from the lower freqs to higher. I tracked down some fm
interference driving my car up and Down street. Strong interference near my
house. It's all these cheap led switching current regulators I am using. I
still have not tried to filter the outputs. I got wiring all over the
place.

Greg

Sun Jan 08, 2012 4:33 am   

On Sat, 7 Jan 2012 23:23:05 +0000 (UTC), gregz <zekor_at_comcast.net>
wrote:


I have to check out my car radio too. I haven't heard any
interference on it, but like I say, it comes at various times. often
from 2 to 9 in the morning (and I don't commute regularly, and even
when I do, I'm only in the car near my house for a couple minutes.)


It's the curse of the electronics age.

>Greg

Sun Jan 08, 2012 5:48 am   

On Sat, 07 Jan 2012 14:51:23 -0800, Jeff Liebermann <jeffl_at_cruzio.com>
wrote:


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.


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. .


Dang, I only rejected one from your whole list.


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.


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

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