Data Sheet
-146-
391. FM Radio Audio Range
Build the circuit to the left, turn on the switch (62) and you will hear some FM
radio stations from the speaker (93) and the heart LED (69) will ash with the
sounds. You may have to press the press switch (61) connected to CH– to get
the FM receiver to scan for a channel. For best FM reception, hold the open end
of the spring wire (9) in the air. Have you ever wondered why there are more
music channels on FM radio stations than in AM radio stations? Because of the
wide 200kHz channel bandwidth, FM radio channels are able to support audio
frequencies up to around 15 kHz. AM radio has much narrower channels (10kHz)
and thus can only support audio frequencies up to around 4.5 kHz. Since music
typically has a lot of high frequency components, it will sound better on an FM radio
station than on an AM radio station.
392. FM Channel Numbering
Build the circuit to the left, turn on the switch (62) and you will hear some FM radio
stations from the speaker (93) and you will see the heart LED (69) light. You may
have to press the press switch (61) connected to CH- to get the FM receiver to
scan for a channel. For best FM reception, hold the open end of the spring wire (9)
in the air. The heart LED (69) acts like an ON-OFF indicator for the circuit. Project
#389 discussed the channel numbering for FM radio. Are you curious why they
started the numbering with channel 200 instead of channel 1? This was to avoid
confusion with TV radio stations which use channel numbering up to 158.
393. FM Radio Wave Propagation
Replace the heart LED (69) with the bi-directional LED (71) in project #392, turn
on the switch (62) and you will hear some FM radio stations from the speaker
(93) and you will see the bi-directional LED (71) light. You may have to press the
press switch (61) connected to CH- to get the FM receiver to scan for a channel.
For best FM reception, hold the open end of the spring wire (9) in the air. The bi-
directional LED (71) acts like an ON-OFF indicator for the circuit. One of the reasons
why FM radio uses the frequency bands 88-108MHz is because of the good radio
wave propagation characteristics in this band. Even ignoring the effects of foliage
like trees and buildings, radio waves propagate further at lower frequencies than
at higher frequencies. In free space (no foliage) it has been shown through the
Friis Transmission Formula that the pathloss (reciprocal of path gain) is given by
(4*p*d*f/c)
2
, where d is the distance between transmitting and receiving antennas,
c is the speed of light and f is the carrier frequency of the signal. This formula clearly
shows that in free space there is much larger pathlosses at higher frequencies.
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