Volvo 2005 C70 - Manual (Page 199)

193
Audio systems, general information
Sending signals
The FM waves do not follow the earth’s
surface nor do they bounce off the atmosphere.
For this reason their range is limited. The AM
waves follow the earth’s surface and reflect
against the atmosphere, giving them a wide
range.
Weak reception (fading)
Because of the limited range of the FM
senders and the fact that these waves are very
reflective, this problem usually occurs with
FM reception. If the sender is blocked by
buildings or mountains, static can result.
Static
The reason why FM but not AM is audible in
covered parking areas, under bridges, etc, is
that FM signals reflect against solid objects
such as buildings. Because these waves are
very reflective, static can result. This static is
the result of the reflected signal and the direct
signal reaching your antenna at slightly
different times causing a cancellation of all
signals. This problem occurs largely in built-
up areas. Your car is equipped with a dual
antenna system which helps alleviate this
problem.
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