Lesson I/Learning Event 5
d. Voice communication. Voice communication is unreliable during high
noise levels encountered on the long-distance transmission path when
conditions are poor. The reason for this is that the voice channel uses a
relatively wide input filter, and the ear is incapable of selecting the
voice signal out of the noise across the voiceband.
e. AM vs FM. The question may be asked, "if FM has an inherent noise-
reduction feature, why not use FM on the HF band?" The answer lies not in
the technology but in the number of users. The HF bands are so crowded that
stations must conduct their communication in relatively narrow bands.
This precludes the use of FM because each FM signal consumes
relatively large segments of the spectrum.
Conversely, the crowded
conditions of the HF band assure the continuation of narrow-band AM on the
HF band. On a comparative basis, an FM station consumes at least 50 kHz for
one voice signal, while a twin-sideband (TSB) AM signal can carry four voice
signals in no more than 12 kHz of spectrum.
Learning Event 5: INTERFERENCE.
1.
Radio interference.
The two most effective ways of overcoming
interference on radio systems is to stabilize the radio transmitter output
signals, and to train operators to respect frequency assignments.
a. Recent advances in radio transmitter design feature frequency
synthesizers.
All frequencies produced by a frequency synthesizer are
stabilized by a master oscillator, and therefore have comparable frequency
stability.
b. Radio operators must select and use only those frequencies that are
assigned by higher authority. They must also operate their sets to minimize
transmission over greater distances than intended by using specific antenna
types and stated output power.
(1) AM signals.
One annoying source of interference between
conventional AM signals is caused by carrier heterodyne.
This means that
when the carriers of two adjacent AM signals heterodyne at radio frequency,
the resulting audio beat after demodulation falls within the bandpass of the
radio receiver output. Single-sideband transmission largely eliminates this
problem because little, if any, carrier power is transmitted.
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