HF (High Frequency) Radio
Introduction to High Frequency Radio Propagation
Activity on the sun can have a wide-ranging effect on the
earth. For example, radiation from the sun removes electrons
from atoms in the upper regions of the earth's atmosphere,
forming the ionosphere. The existence of the ionosphere allows
the use of High Frequency (HF) radio as a means of communication
over long distances.
Does HF Radio Work Over Long Distances?
An HF signal transmitted from the earth may travel some way
through the ionosphere before being "bent" back
down towards the ground. This occurs due to the interaction
between the HF signal and electrically charged particles in
the ionosphere. The signal can then "bounce" off
the ground back into the ionosphere, return to the earth again,
and so on. The distance a given HF signal will travel depends
on the frequency, transmitter power, take-off angle relative
to the ground and the state of the ionosphere through which
it is travelling.
given distance and time, there will be a certain range of
HF frequencies that are most likely to provide successful
communications; frequencies outside that range will work poorly
or not at all. Simply increasing the power of an HF signal
will not help if the frequency is too high for the distance
required. Increasing the power may help if the frequency is
too low, but using a higher, more suitable frequency is the
best option. The highest frequency which may be used for reliable
HF communications is known as the Maximum Usable Frequency
Do Conditions Affecting the Use of HF Radio Vary Over Time?
Extreme Ultraviolet (EUV) radiation from the sun creates the
ionosphere. The EUV radiation arises from the bright and hot
regions which overlie sunspots (areas of strong magnetic fields
on the sun's surface). As the sun progresses through its eleven
year cycle of activity, the number and size of sunspots will
vary, as will the level of EUV radiation. Changes to the ionosphere
that result from this mean that conditions affecting the use
of HF radio will also change over the solar cycle.
low point of the solar cycle, only the lower frequency HF
signals can be transmitted over a given distance. At the peak
of the cycle, the higher frequencies in the HF band can be
transmitted over the same distance. Other factors important
in determining the range of usable HF frequencies include
the seasons, the time of day and the relative locations of
the transmit and receive points.
Kind of Disturbances Can Degrade HF Communications?
Short-Wave Fadeouts - short lived (up to two hours) disturbances,
in which solar flare activity results in the absorption of
lower frequency HF signals. These will only affect signals
passing through the daylight ionosphere.
Storms - large scale changes in the chemical composition of
the ionosphere resulting in changes to the MUF. Decreased
MUFs restrict the frequencies available for use over a given
distance. Ionospheric storms normally last for one to two
Selective call - SelcallSelective call - Selcall
- provides a simple and efficient method of calling stations
within a network. With the combined Selcall - Telcall
option fitted - all current derivatives of CCIR 493 format
can be programmed into the your Selcall equipped transceiver
on a channel by channel basis.
more comprehensive information on HF propagation: click
material has been sourced from I.P.S. Radio & Space Services