In article ,
micky wrote:

In the last six months I've had interference with two FM radio
stations. I thought FM was immune to most interference?? So
what might be doing it?

FM is less vulnerable than AM, to impulse noise (static).

It's still vulnerable to both broadband noise, and narrowband noise,
if the strength of the noise signal comes close to (or becomes greater
than) the strength of the desired signal.

Most FM radios are also vulnerable to "desense" - a strong-enough
signal of any sort, located anywhere in the radio's RF passband (which
can be much wider than the FM band that you can tune) can saturate the
first stage or two of the radio's RF section. This has the effect of
making the radio less sensitive to the signal you want to hear - it's
as if the desired signal "fades out".

It's so loud I can't hear the program,. But when I hold my fist very
near the radio or touching the front, the noise goes away gradually
and then completely when my hand is close enough. Is there some
way to duplicate the effect of my hand being near the radio???????
I tried a lot of things in a very different stituation and nothing I
tried worked.

The fact that the interference fades out when you bring your hand near
the radio, makes me suspect the following:

(1) When the radio is being used normally, it's not actually getting a
strong signal from the station you want to hear.

(2) The interfering signal isn't actually that strong, either - but
when it gets into the radio's RF section it's at least as strong
as the signal you want to receive.

(3) When you hold your hand near the radio, your body is acting as an
auxiliary antenna - it's picking up and conducting the radio
station's signal on your skin surface. The signal is then being
"coupled" into the radio's antenna from your hand - capacitive
coupling.

(4) Your body is not acting as an efficient antenna for the
interfering signal, which suggests that the interference might
*not* be getting into the radio via the antenna, but via another
pathway (possibly the power line).

I live in Baltimore and it occurs on 88.5 and 90.1 MHz. FWIW these
are both DC stations 30 or 40 miles away, NPR and C-SPAN raido.. 88.1
which is local never gets interference. I don't know where other
stations on the dial are,located sine these are the only 3 I listen
to.

This is consistent with the problem being one which affects only weak
signals. 30 - 40 miles is a good distance for FM (it's probably
"medium fringe"), and stations such as NPR are likely to be smaller
(lower power) than big commercial stations.

Modern buildings often have a lot of metal in their structure,
reflective metal coatings on their windows, etc. - which tends to
weaken the FM signals pretty badly. I'm somewhat surprised that you
can get a decent-quality FM signal indoors from 30-40 miles away.

I think there are three approaches you can take to identifying the
problem:

(1) Locate the source of the interference, and either shut it down or
filter it at the source.

Switching off appliances and breakers, as has been suggested, is a
great way to do this... probably the easiest.

If the source is outside your house, you may need to go on a
"T-hunt" (transmitter hunt). A local ham-radio operator might be
able to help you with this... T-hunting is one aspect of the
hobby, and quite a few hams have directional antennas and the
skills needed to use them.

If you can find the source, but can't afford to shut it down
entirely, then connecting a plug-in RFI filter between the
appliance and the power line would probably help a lot.

(2) Filter the interference at your radio. The behavior you are
seeing makes me suspect that it may be getting into the radio via
the AC power cord. Adding a good plug-in RFI filter at the wall
outlet might choke off the interference before it gets into the
radio.

(3) Try to improve the strength of the radio signal that's getting to
your radio. Ideally, put up a simple outdoor antenna, and run a
coax cable to the radio. If the radio doesn't have an
external-antenna jack, you might be able to connect the coax to
the radio with a simple clip-lead jumper (from the coax center to
the radio's existing antenna), or connect two wires to the center
and braid of the coax and stretch these in opposite directions
near the radio.

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