Ford Probe Owner's Manual page 144

NOTE: Always make sure your antenna
(whether you have a power antenna or
a manually adjustable antenna) is fully
extended to the maximum length for
proper reception. If your antenna is not
fully extended, you may experience
signal loss while traveling in fringe
reception areas.
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.
Repeated pops and hisses which are heard
during an otherwise clear broadcast can occur
near the station because of the "line of sight"
characteristic of FM radio waves.
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.
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