[OT] Locating source of FM radio interference

[Bill Freeman]

Michael ODonnell writes:
 > 
 > Something near our house has recently started
 > generating spectacular amounts of radio intereference
 > that's most noticeable around 89MHz.  I have no
 > portable radio equipment of any kind except a humble
 > little $10 handheld with a normal telescoping antenna
 > that seems not to be very directional, or at least I
 > don't understand its directionality.  Is there some
 > way I can use it to do some sort of triangulation on
 > the source of interference?  Maybe some particular
 > way of holding/orienting it, or selectively/partially
 > shielding it, or tuning it, or...  ?

	A particular problem with an FM receiver is that its response
to signal strength may seem counter intuitive.  As the desired signal
gets weaker, it doesn't get quieter (at least until the very end), but
rather noise gets louder.  Of course, your particular "noise" might be
serving as a signal, rather than the "white" or "background" noise
that indicates weak signal.  If your portable also receives AM, and if
the interference is broadband enough to be received in the AM mode,
then the results may be easier to interpret.  AM antennas in portables
also tend to be fairly directional, though in the sense of having a
bilateral null, rather than a peak.

	In either case, however, a strong enough (interfering) signal
will give no audible change in response over a fairly broad range of
signal strengths (unless it has an "S meter").  For work close to the
source, then, you need a means of seriously attenuating the signal.
Your portable is unlikely to come equipped with one, so you have to
fabricate your own.  This probably takes the form of a shield with
an aperture.

	If you have one of those metalized plastic bags in which you
are supposed to put your speed pass if you don't want to use it at the
moment, that's a good start.  Put the portable inside, fold over the
closure a few times, and see if it behaves like a radio between
stations.  If this isn't enough, try a metal foil bag (though making a
good connection where you want to edges to act like a continuous side
could be as difficult as making the bag hold water).  If this works
then partially open the bag, making the opening progressively larger,
until you hear the interference.  Then keep the opening that size
while you walk around to try to get a feel for a constant signal
strength contour.  This may give you a better idea of the source
location.  You may need to repeatedly close down the aperture and make
another contour for a greater signal strength as you close in.

	Note, however, that the spot you find may be, rather than the
source, a piece of metal that is coupled to the actual source (fence,
roof flashing, rain gutter, phone line, power line, etc).  Coupling
can be via actual electrical contact, such as a bad cordless telephone
base injecting noise into the phone line, or simply by proximity.
Being able to work in both AM and FM modes can be valuable here, since
coupling may be quite different between the two.  A good test is to
have someone not holding the portable in the bag touch the fence,
etc., (but not a power line), which should make a marked difference in
the signal.

	Finally, if you can track it to one or a few dwellings, and if
you have the social skills to persuade the owner to unplug things one
at a time, you may not need to pinpoint things more closely.  If there
is some new appliance that is causing the problem, the manufacturer is
usually legally bound to fix the issue, though this is still a pain.

							Bill