Skanti TRP 9250 S, Техническое руководство

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PROPAGATION OF HF AND MF RADIO WAVES
Introduction
The TRP 9250 is able to operate on frequencies from 1.605 MHz to 30 MHz. This frequency
span covers the upper part of the MF range (0.3 - 3 MHz) and the whole HF range (3 - 30 MHz).
In this wide frequency area the propagation properties of radio waves differs markedly from one
edge to the other.
The propagation in the MF and HF frequency bands takes place in two different ways, namely
one in which the radio waves follows the surface of the Earth ("Groundwaves") and one in which
the radio waves are reflected from the Ionosphere ("Skywaves"). Both propagation modes are
influenced by many factors, the major being the Sun, the position of the vessel on Earth and the
time of day.
Groundwave propagation
In the MF band and the lower part of the HF band, at frequencies up to 5 MHz the predominant
propagation mode is groundwave propagation.
In this mode the waves originating from the transmitter antenna will follow the Earths curvature to
the receiver antenna. It is not necessary to have direct line of sight between the antennas as it is
in the case of VHF radiotelephones. Under normal conditions the range of communication can be
expected to be up to 800 km at 1.6 MHz decreasing to about 500 km at 5 MHz. The reason for
the shorter range on the higher frequencies is that the attenuation in the atmosphere increases
with the frequency and at the same time the radio waves’ ability to follow the curvature of the
Earth decreases.
The time of day will also influence on the propagation. During the day, the atmosphere, due to
the radiation from the Sun, will be more absorptive than during the night when a range of up to
1000 km can be expected.
MF band radio waves are radiated with the highest efficiency from a long antenna. This is due to
the long wavelength of the signal.
At frequencies above 5 MHz the radio waves ability to follow the Earth’s curvature decreases and
at 30 MHz the propagation is almost along a straight line. Therefore if ground wave propagation
is to be used on 30 MHz the range is confined to distances at which there will be optical sight
between the antennas in question.
PROPAGATION OF HF AND MF RADIO WAVES OPERATION
TRP 9250 TECHNICAL MANUAL 2-55