Overview of HF operation
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 is
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.
HF propagation
Radio wave propagation illustrated
BARRETT 950 HF TRANSCEIVER
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