1: Introduction
1-4 PRC1099A-MSOP
increase in efficiency, as the carrier uses 2/3 of the power and carries no useful
information; the two sidebands contain identical information. This means that
the SSB transmitter puts all of the power into one information carrying
sideband. This makes the PRC1099A as effective as a 120W AM transmitter.
Apart from the improved efficiency, the SSB signal occupies less than half the
frequency space of the AM signal and permits much better utilization of the
crowded HF spectrum. The SSB signal is more intelligible at poor signal
levels and is far less affected by selective fading and interference. To receive
SSB, it is necessary to use a special receiver that locally reinserts the
suppressed carrier so that the transmission may be demodulated. The SSB
signal is unintelligible when received by a standard AM receiver.
HF
Communications
The HF communications spectrum is primarily for long distance
communications, while VHF is favored for local communications. If the
correct frequencies and antenna systems are used, the HF spectrum will
provide effective communications over almost any distance including inter-
continental ranges. There are two main modes of propagation of HF signals–
ground wave and sky wave. The ground wave follows the surface of the earth
and provides reliable signals over short ranges. The signals are attenuated
very rapidly as they pass over the surface of the earth, so that high powers and
good antennas are essential for good ground wave coverage. The ground wave
attenuation increases as a function of frequency, and the lower frequencies
below 3 MHz are favored for ground wave operation. This mode may be the
only effective method for local coverage in areas too mountainous for VHF
and UHF operation.
Most HF communication is by sky wave where the signals are refracted by the
reflecting layers of the ionosphere. Long distances can be covered with little
signal attenuation, provided the correct frequency is chosen. The ionosphere
does not stay constant; it varies with the time of day, time of year, the sun spot
cycle and the activity of the sun. Solar flares can cause complete radio
blackouts with little warning. HF communications are affected by static
caused by lightning; sometimes from storms many hundreds or thousands of
kilometers away. A clear channel can never be assured, as long distance
propagation may cause strong interfering signals on the frequency from great
distances. It must always be understood that although long distance
communications are possible with low powers and simple antennas, high
reliability and freedom from interference is not possible. In spite of the
problems, a surprisingly good standard of communications can be achieved
provided that care is taken to select the correct frequency and to use good
equipment with an efficient antenna system.
The correct choice of frequency is beyond the scope of this manual and may
be limited by the available frequency allocations. The following information
provides a starting point for choosing the best frequency. Remember that the
final guide should be an actual test, as often only a small change in frequency
may make a big difference in signal strength.
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