In article ,
micky wrote:

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.)

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.

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.

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 AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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