path - by Eddie Runner (NU5K)
FM radio sounds scratchy 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 radio 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 make 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.
THIS IS MULTIPATH!!
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 automatically reducing the
receiver to MONO and some even have multiple antennas (usually in the
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.