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Placing FM antennas on high sites, such as towers on top of buildings, or
mountains can increase this distance. The limited reach of FM is why public
and commercial broadcasters use more than one FM frequencies for
transmission, as it is impossible to cover a large geographical area with a single
FM transmitter.
AM signals are able to cover much greater distances. The lower frequencies
that are used for AM are effectively reflected back to earth by the earth’s
atmosphere. This is called the skef wave effect. In addition, AM signals are
conducted through the ground, called the ground wave effect. These two effects
combine to give AM a much greater reach than FM.
The diagram below shows the differences in coverage area for an antenna
broadcasting AM and FM.
The propagation of the sky wave and the limitations of line of sight from an antenna
Demodulation
The radio receiver is used to reverse the process of modulation. The reverse
process is called demodulation. This allows the original modulating wave (the
audio signal) to be retrieved from the modulated radio wave.
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Community Radio Technical Manual