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HF (High Frequency) is the radio spectrum with frequencies between
1.6 and 30MHz.Within this radio spectrum an efficient form of transmitter
modulation, SSB (Single Side Band), is used. This, combined with the
use of the ionosphere - a layer of ionisation gases that resides between
100 and 700km above the earths surface, provides efficient, cost effective
communications over short, medium and long distances - without the need
for expensive re-transmission devices, such as the VHF or UHF repeaters
or satellites, all of which have on going operational costs and a reliance
on a physical infrastructure.
In many remote areas, HF/SSB is the only form of communication possible.
When HF/SSB radio waves are generated by the transceiver there are
usually two components:-
- The ground-wave, which travels directly from the transmitting antenna
to the receiving antenna following the contours of the earth.
-
The sky-wave, which travels upward and at an angle
from the antenna, until it reaches the ionosphere (an ionised layer
high above the earth's surface) and is refracted back down to earth,
to the receiving antenna.
Generally speaking, ground-wave is used to communicate
over shorter distances usually less than 50km. Because ground-wave follows
the contours of the earth, it is affected by the type of terrain it
passes over. Ground wave is rapidly reduced in level when it passes
over heavily forested areas or mountainous terrain.
Sky-wave is used to communicate reliably over medium to
long distances up to 3,000km. Whilst the nature of sky-wave propagation
means it is not affected by the type of terrain as in ground waves it
is affected by factors involving the ionosphere as described below.
The following illustrations show the characteristics of
ground-wave and sky-wave propagation during day and night time. In each
illustration the height of the ionosphere above the ground is shown.
In both illustrations Station A communicates with Stations
B, C and D.
Propagation from Station A to B is by ground-wave. The
diagrams illustrate that the ground wave is not affected by the time
of day and the height of the ionosphere above the ground.
Propagation from Station A to C and D, however, is by
sky-wave and as the diagrams illustrate the sky wave is significantly
affected by the time of day and the height of the ionosphere above the
ground.
Under each diagram there are recommended working frequencies
listed. Please note that these will vary according to time of year and
other factors. They are intended only as a guide and are subject to
change.
Day
The sun is higher, the ionosphere is higher, the best frequency to
use is higher
A to B - Possible optimum working frequency is 3 MHz
A to C - Possible optimum working frequency is between 7 - 9 MHz
A to D - Possible optimum working frequency is between 13-16 MHz
Night
The sun is lower, ionosphere is lower, best frequency to use is lower
A to B - Possible optimum working frequency is 3 MHz
A to C - Possible optimum working frequency is between 5 - 7 MHz
A to D - Possible optimum working frequency is between 9 -12 MHz
Factors which affect HF/SSB communications
There are a number of different factors, which will affect
the success of your communications via HF/SSB radio. These are outlined
below:-
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Frequency selection
Frequency selection is perhaps the most important
factor that will determine the success of your HF/SSB communications.
Generally speaking the greater the distance over which you want
to communicate, the higher the frequency you should use. Beacon
call, a Selcall (selective call) function built into the Barrett
950 transceiver, makes finding the correct frequency to use easy.
Beacon call is based on the network transceivers all having a selection
of frequencies that will accommodate most ionospheric conditions.
When in standby the network transceivers scan these frequencies
waiting for a call (Selcall or beacon call) from another transceiver.
The transceiver wishing to check for the best frequency to operate
on sends a Beacon Call to the station he wishes to contact. If his
call to the other station is successful he will hear a revertive
call from the station he is calling, indicating the channel he selected
was suitable for the ionospheric conditions prevailing. If he does
not hear this revertive call or it is very weak, he tries on another
channel until a revertive call of a satisfactory signal strength
is heard.
-
Time of day
As a rule, the higher the sun, the higher the frequency
that should be used. This means that you will generally use a low
frequency to communicate early morning, late afternoon and evening,
but you will use a higher frequency to cover the same distance during
times when the sun is high in the sky (e.g. midday). You will need
to observe the above rule carefully if your transceiver has a limited
number of frequencies programmed into it, as you may only be able
to communicate effectively at certain times of the day.
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Weather Conditions
Certain weather conditions will also affect HF/SSB
communications. Stormy conditions will increase the background noise
as a result of static caused by lightning. This background noise
could rise to a level that will blank out the signals you are trying
to receive.
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Man-made electrical interference
Interference of an electrical nature can be caused
by overhanging power lines, high power generators, air-conditioners,
thermostats, refrigerators and vehicle engines, when in close proximity
to your antenna. The result of such interference may cause a continuous
or intermittent increase in the level of background noise.
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System configuration and installation
The method in which your system is configured and
installed will also affect the success of your HF/SSB communications.
Your choice of antenna system and power supply is critical. Correct
installation is also extremely important. An HF/SSB transceiver
is generally installed using different rules to those used to install
VHF or UHF transceivers. Failure to correctly install an HF/SSB
system will greatly affect the communications quality you will obtain.
Special note - HF communications compared with VHF or UHF short
distance communications
Communications on any HF/SSB transceiver will sound different
to that on a VHF (Very High Frequency) radio or UHF (Ultra High Frequency)
radio or telephone. This is because of the nature of HF propagation
and the modulation methods used. On HF/SSB transceivers there will always
be background noise evident behind the signal you are receiving and
this will increase when there is electrical interference or thunderstorm
activity in the area.
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