Sometimes FM radio sounds
staticy and bad because of multipath interference. Multipath (often called
picket fencing or flutter) happens when FM signals bounce around between the
buildings in a city, or other large obstructions, this bounce causes a
reflection and your FM radio tries to lock onto the original signal as well
as the reflection! This phenomenon occurs mostly near the buildings and
worse between the buildings, but sometimes even fairly far away from the
buildings. The static is more noticeable when moving but it is possible to
stop your car in a path, if you do stop in a path you can usually ease your
car forward and the reception will change, this is a telltale sign that it
is indeed multipath. You can also drive down the road till you hear a
flutter and then go around the block and come back to the exact same spot
and the flutter will be exactly the same and exactly the same spot.
To identify
multipath first try the AM radio and make sure the AM is strong, if the
antenna has any problems they will usually cause the AM to be terrible or
weak. Assuming the AM sounds fine then the antenna is good. Next step is to
tune the FM radio and see if it sounds normal, typically multipath wont be
very noticeable while sitting still, if the stations sound very bad be sure
to try many stations down the dial. If all stations sound bad, then of
course the receiver may have a problem (might be broke!). If the stations
sound fine or some bad and some good then the next step is to drive the car
through some areas where the static occurs and then round the block and go
past the same spot with the same station and verify it always occurs at that
spot. This will verify there are places you can drive where that radio
station sounds fine, and places where that station sounds bad. DEFINATLY
MULTIPATH.
To get rid of
multipath static may not be easy, most folks would rather live with it than
go to a lot of trouble to get rid of it. For those that cant stand it though
the secret is to reduce the sensitivity of the set so that it no longer
tries to lock on the reflected signals, only the main signal. Reducing the
sensitivity might be as simple as pushing your antenna down a ways. Some
factory car stereos reduce the multipath by automaticly reducing the
receiver to MONO and some even have multiple antennas (usually in the
windshield)…
Multipath I have found over the years bothers folks very badly because they think their receiver is busted, but once multipath is explained and they realize there are only a few spots in the average city where the flutter occurs as they drive, they get to know the bad spots they drive through and feel comfortable in that their radio isn’t really busted.
