Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 392 08 5(47)
FM Band
W3003002
FM band signal. (100 km = 62 miles)
Radio signals in the FM band have higher frequency
and shorter wavelengths than the AM band. Some
characteristics of the FM band include:
•
FM is able to eliminate atmospheric noise mainly due
to its method of propagation. Atmospheric noises are
generally amplitude related and have little effect on
FM signals.
•
Reflect off of obstacles.
•
Penetrate the ionosphere.
Radio frequencies over 30 MHz are effectively absorbed
by the earth, eliminating the ground wave. Even when out
of a direct line of sight, the signal may be reflected into
areas that would be in a “shadow” otherwise.
FM signals and light rays respond to atmosphere in a
similar manner. The atmosphere bends them both so they
will follow the curvature of the earth to a certain extent.
This is called refraction and it is similar to a lens which
can bend light waves. A good example of this is when
you see a sunset. The sunlight is visible with the sun well
below the horizon. FM signals characteristically have a
shorter range than AM signals.
Factors which affect the line of sight include:
•
Height of the broadcast antenna.
•
Height of the receiving antenna.
•
Terrain and buildings in the broadcast path.
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