Ford Grand Marquis Owner's Manual page 65

2. Terrain
The terrain (hilly, mountainous, tall buildings) of
the area over which the signal travels may
prevent the FM signal from being noise-free.
If there is a building or large structure between
the antenna and station, some of the signal
"bends" around the building, but certain spots
receive almost no signal. Moving out of the
"shadow" of the structure will allow the station
to return to normal.
When the radio waves are reflected off objects or
structures, the reflected signal cancels the normal
signal, causing the antenna to pick up noise and
distortion. Cancellation effects are most
prominent in metropolitan areas, but also can
become quite severe in hilly terrain and
depressed roadways.
To minimize these conditions, a stereo/mono
blend circuit has been incorporated into this
system. This feature automatically switches a
weak stereo signal to a clearer monaural signal,
which improves the quality of reception.
Several sources of static are normal conditions
on AM frequencies. These can be caused by
power lines, electric fences, traffic lights and
thunderstorms.
Another reception phenomenon is Strong Signal
Capture and Overload. This can occur when
listening to a weak station and when passing
another broadcast tower. The close station may
capture the more distant station, although the
displayed frequency does not change. While
passing the tower, the station may switch back
and forth a few times before returning to the
original station.
63