Ford Mercury Villager 1997 Owner's Manual page 54

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.
This condition exists 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
becomes quite severe in hilly terrain and depressed roadways.
To minimize the effects of 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.
When several broadcast towers are present (common in
metropolitan areas), several stations may overload the receiver,
resulting in considerable station changing, mixing and distortion.
Automatic gain control circuitry for both AM and FM bands
has been incorporated into this system to reduce strong signal
capture and overload.
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