Radio Reception; Operating Tips for Audio system- Page 74

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Audio Operation

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Radio Reception

AM characteristics

AM signals bend around such things as 
buildings or mountains and bounce off the 
ionosphere. Therefore, they can reach 
longer distances than FM signals. Because 
of this, two stations may sometimes be 
picked up on the same frequency at the 
same time.

FM characteristics

An FM broadcast range is usually about 40–
50 km (25–30 miles) from the source. 
Because of extra coding needed to break the 
sound into two channels, stereo FM has 
even less range than monaural (non-stereo) 
FM.

Signals from an FM transmitter are similar to 
beams of light because they do not bend 
around corners, but they do reflect.

Unlike AM signals, FM signals cannot travel 
beyond the horizon. Therefore, FM stations 
cannot be received at the great distances 
possible with AM reception.

Atmospheric conditions can also affect FM 
reception. High humidity will cause poor 
reception. However, cloudy days may pro-
vide better reception than clear days.

Multipath noise

Since FM signals can be reflected by 
obstructions, it is possible to receive both the 
direct signal and the reflected signal at the 
same time. This causes a slight delay in 
reception and may be heard as a broken 
sound or a distortion. This problem may also 
be encountered when in close proximity to 
the transmitter.

Operating Tips for Audio system

Station 2

Station 1

Ionosphere

FM Station

40—50km
(25—30 miles)

AM wave

FM wave

FM wave

100—200 km (60—120 miles)

Ionosphere

Reflected wave

Direct