[OT] Locating source of FM radio interference

[Brian Chabot]

Bill Freeman wrote:

>	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.
>  
>

I used to do tracking on radio signals years ago...

There is a technique that works without going to RadShack or the local
supermarket  for parts...  Though, as has been pointed out, it works
better for AM than FM... but it should work in FM.

It's called "Body Shielding".  Your body can act as a shield.  Tune the
radio so you can hear the interferance, but *just* barely.  Put the
(probably not-fully-extended) antenna up against your chest and slowly
turn around, listening carefully.  You are looking for the absolute
*worst* reception where you can still make out a signal. If you do it
right, there will be a direction in which the signal fades out.  At that
point, the source is likely to be right behind you.  This is obvious in
AM signals, but probably pretty subtle in FM... You have to tune
off-frequency till you barely hear the signal. 

>	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.
>

Excellent point.  I used to have a hell of a time with transmitters near
train tracks.


Brian