Critical Frequency, LUF, and MUF
- the Critical Frequency, Lowest Useable Frequency, LUF, and Maximum Usable
Frequency, MUF used in radio communications and radio signal propagation
When looking at ionospheric or short wave radio signal propagation for
planning a radio communications network or system, or when predicting the HF
propagation conditions, there are several frequencies that are important, and
are often mentioned in radio signal propagation prediction programmes and in
other literature associated with signal propagation. The frequencies include the
Critical Frequency, the Lowest Useable Frequency (LUF), and the Maximum Usable
Frequency (MUF). Their definitions are at the centre of determining which
frequencies will provide the optimum performance for the radio communications
system or network.
Critical Frequency
The critical frequency is an important figure that gives an
indication of the state of the ionosphere and the resulting HF propagation. It
is obtained by sending a signal pulse directly upwards. This is reflected back
and can be received by a receiver on the same site as the transmitter. The pulse
may be reflected back to earth, and the time measured to give an indication of
the height of the layer. As the frequency is increased a point is reached where
the signal will pass right through the layer, and on to the next one, or into
outer space. The frequency at which this occurs is called the critical
frequency.
The equipment used to measure the critical frequency is called an ionosonde.
In many respects it resembles a small radar set, but for the HF bands. Using
these sets a plot of the reflections against frequency can be generated. This
will give an indication of the state of the ionosphere for that area of the
world
LUF - Lowest Usable Frequency
As the frequency of a transmission is reduced further
reflections from the ionosphere may be needed, and the losses from the D layer
increase. These two effects mean that there is a frequency below which radio
communications between two stations will be lost. In fact the Lowest Usable
Frequency (LUF) is defined as the frequency at below which the signal falls
below the minimum strength required for satisfactory reception.
From this it can be seen that the LUF is dependent upon the
stations at either end of the path. Their antennas, receivers, transmitter
powers, the level of noise in the vicinity, and so forth all affect the LUF. The
type of modulation used also has an affect, because some types of modulation can
be copied at lower strengths than others. In other words the LUF is the
practical limit below which communication cannot be maintained between two
particular radio communications stations.
If it is necessary to use a frequency below the LUF then as a
rough guide a gain of 10dB must be made to decrease the LUF by 2 MHz. This can
be achieved by methods including increasing the transmitter powers, improving
the antennas, etc..
MUF - Maximum Usable Frequency
When a signal is transmitted using HF propagation, over a
given path there is a maximum frequency that can be used. This results from the
fact that as the signal frequency increases it will pass through more layers and
eventually travelling into outer space. As it passes through one layer it may be
that communication is lost because the signal then propagates over a greater
distance than is required. Also when the signal passes through all the layers
communication will be lost.
The frequency at which radio communications just starts to
fail is known as the Maximum Usable Frequency (MUF). It is generally three to
five times the critical frequency, dependent upon the layer being used and the
angle of incidence.
Optimum working frequency
To be able to send signals to a given location there are
likely to be several different paths that can be used. Sometimes it may be
possible to use the either the E or the F layers, and sometimes a signal may be
reflected first off one and then the other. In fact the picture is rarely as
well defined as it may appear from the textbooks. However it is still possible
to choose a frequency from a variety of options to help making contact with a
given area.
In general the higher the frequency, the better. This is
because the attenuation caused by the D layer is less. Although signals may be
able to travel through the D layer they may still suffer significant levels of
attenuation. As the attenuation reduces by a facto of four for doubling the
frequency in use this shows how significant this can be.
Also by increasing the frequency it is likely that a higher
layer in the ionosphere will be used. This may result in fewer reflections being
required. As losses are incurred at each reflection and each time the signal
passes through the D layer, using a higher frequency obviously helps.
When using the higher frequencies it is necessary to ensure
that communications are still reliable. In view of the ever-changing state of
the ionosphere a general rule of thumb is to use a frequency that is about 20%
below the MUF. This should ensure that the signal remains below the MUF despite
the short-term changes. However it should be remembered that the MUF will change
significantly according to the time of day, and there fore it will be necessary
alter the frequency periodically to take account of this.
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