Volvo Trucks North America, Inc. Date Group No. Page
Service Bulletin 4.2004 392 08 4(47)
AM Band
W3003003
AM radio signals. (100 km = 62 miles)
Radio signals in the AM band have lower frequency
and longer wavelengths than the FM band. Some
characteristics of the AM band include:
•
The frequency used is relatively low; therefore, these
waves can bend around obstacles and skip along the
ground.
•
The waves are reflected by the ionosphere layer in the
atmosphere.
•
Long range reception is sometimes available.
•
AM is subject to electrical interferences.
•
AM can be blocked by steel and/or concrete structures.
The AM frequencies have longer range due to the ground
wave. The ground wave follows the curvature of the
earth and is effected by its conductivity. The greater the
conductivity, the less the signal loss; so transmission
over water is better than over land. The ground wave in
this frequency band has a range of 80–320 kilometers
(50–200 miles).
The sky wave will pass through the atmosphere and
reflect off of the ionosphere to the vehicle’s antenna. At
night, the ionosphere is actually higher than during the
day, so it is possible for AM signals from great distances to
be received during nighttime hours. As a result, many AM
stations actually reduce transmitting power in the evening
hours to maintain the same coverage as during the day.
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