In article ,
micky wrote:
Interference with FM radio, goes away when my hand is near.

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?

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.

Details.
It only happens sometimes, often in the middle of the night, and when
it's on one radio it's on two others also. Two have digital tuning
and one, maybe two, have analog (One digital is expensive 25 y.o,
the other maybe 10, one analog is transistor 40 y.o. or so. All of
them work well otherwise.) It often lasts only 15 minutes or less,
sometimes with interruptions with no noise. It may last longer other
times but I don't have the patience to keep checking. When I find
my mp3 player with FM radio, I'm going to walk around the n'hood and
see if I can find the source.

There's a bus with a 2-way radio that parks less than 100 yards from
my bedroom, but the noise occurs even when it's not thjre. My n'bors
all seem to keep regular hours, not up in the middle of the night,
and none have SWave antennas visible, and the noise has no words.

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.

It sounds like a foghorn, a consistent deep (loud) tone, but a little
higher pitch than a real foghorn. .

There is an older aviation navigation system, VOR, that runs in the range
just above the FM broadcast band (108-118 MHz). It works by broadcasting
a couple of tones at a low frequency, which, when compared, gives aircraft
a compass heading of their location relative to the airport that it's
located at.

It could be an RF Image. The usual IF for a FM receiver is 10.7
MHz, and the local oscillator is usually above the frequency, so a
super-heterodyne receiver can also be sensitive to signals 21.4 MHz
above the tuned station.

It might be a nearby airport, or it could be at a farther distance
than you would expect, especially now with the sunspots kicking up and
strange radio propagation effects can occur.

(The interference can go the other way, too, where the radio's local
oscillator will leak out and jam a VOR signal 10.7 MHz above the tuned
station's frequency. This is the main reason that personal electronics
are supposed to be turned off on aircraft at various times. Now, largely,
an obselete problem).

Mark Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)