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.