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Bredhurst Receiving and Transmitting Society |
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Syllabus Sections:- 6b Ionosphere 6b.1 Understand that the ionosphere comprises layers of ionised gasses and that the ionisation is caused by solar emissions including ultra-violet radiation and charged solar particles. Recall the ionospheric layers (D, E, F1 and F2) and approximate heights. FACT : Ionisation only takes place during the daytime. Why ? Because that is the time when the sun can be shining !! It is the effect of the sun's radiation which ionises the layers of gasses in the ionosphere. In the Intermediate Licence Course you saw the following chart. Note that the chart only shows layers D E & F.
For this Advanced course you need to know that the F layer during the daytime divides into two layers F1 and F2 and recombines again at night. Below is an updated chart giving more information than previously shown above.
The ability for your radio waves in the frequencies 1 to say 70MHz to reach long distances is due to a single or several reflections from the "ionised" layers and then the earth's surface using the F layer at night and the F1 and F2 layers during the daytime. The gasses of the F1 and F2 layers is ,during daylight, partially ionised by the sun's rays (radiation), that is that some of the molecules of air are converted into ions and free electrons and at night there is a slow recombination of the ions and electrons which then form the F layer. Nothing is certain about the amount of ionisation that occurs. The amount of ionisation caused by solar emissions including ultra-violet radiation and charged solar particles has the following variation :-
6b.2 Recall that the E layer can refract radio waves and that sporadic-E is caused by areas of highly ionised gas that can refract waves in the VHF band. Recall that the E layer supports single hops up to about 2000km. Is it refraction or reflection ? It is the refraction of the radio waves as they travel through changing densities of ionised gases that cause the radio wave to be bent and this can occur to such an extent that the signals are refracted back to earth. With this amount of refraction it appears as if the wave are reflected. So what appears to be reflection is many refractions or bending the radio waves.
This same bending effect happens at all changes in densities whether it is a glass prism where there is an air to glass layer, or in a pond where there is an air to water layer. The bending effect occur at the separation layer of densities and this may happen hundreds of times in the ionosphere. Just like water there will come a time when the signal does not continue through the densities change layer but is in fact reflected and this is called "total internal reflection" and is the point at which the radio wave starts its return journey to earth. During the summer months regions of intense ionisation of the E layer of the ionosphere, which allows much higher frequencies to be reflected (up to 150 Mhz) which normally pass straight through the ionosphere. The SPORADIC E season can open up the 50, 70, and 144 Mhz bands to some rare Dx with extremely strong signals but deep fading (QSB) so the length of the QSO must be kept to a minimum. The sort of distances that E layer propagation can achieve is about 2000 kms. 6b.3 Recall that the F2 layer provides the furthest refractions for HF signals (about 4000km) and that the F layers combine at night. Recall that multiple hops permit world-wide propagation. It is the F2 layer that provides the greatest distance at about 4000 kms as it is the highest level of the ionosphere that aids propagation. See the diagram below.
From the diagram you can see that the distance covered by reflection from the lower F1 layer is less than the higher F2 layer for the same radio signal transmission. It is by multiple hops, reflections between the ionosphere and the ground or sea that enables worldwide communication on the HF bands. 6b.4 Understand how fading occurs and its effect on the received signal. There are two types of fading that can affect radio signals :- Fade outs :-
Fading
Fade outs Sid (Sudden Ionospheric Disturbance) is apparent from sudden disappearance of signals from a few minutes to several hours DURING DAYLIGHT HOURS ONLY. These occurrences are found to be due to eruptions on the sun. These affect frequencies below 30MHz. The second type of fade out happens when ionospheric storm takes place. At this time the F layer's ionisation is much reduced and the height above ground varies quite considerably. The ionospheric storm originates as the same time as the faster moving radiation which causes sid and can take two to three days to reach the IONOSPHERE. With them come magnetic storms and this affected the earth's magnetic field. The onset of the ionospheric storm is slow ,taking several hours to reach a maximum and the resulting fades outs can last for about two days but diminish in intensity. Fading This general fading is apparent when operating with propagation through the ionosphere and through the troposphere. This type of fading as opposed to the fades out is used to describe the rapid variation in signal strengths received with periods of a few minutes to a few seconds or less. Interference fading Interference fading is due to two or more signals from the same transmitter being received that have traveled by differing routes. Thus the signal received is a resultant signals from all the various paths. At times the resultant signal is aided by other signals received but at other times the signal is degraded by another signal which leads to significant changes in received signal strength level. Polarization fading Polarization fading is when the radio waves are passing through the ionosphere they are being constantly changes in their polarization especially by the earth's magnetic field. When the signal is of a different polarisation to that of the receiving antenna there will be significant loss of signal strength. Absorption fading Absorption fading is due to various amounts of absorption which take place in the ionosphere and troposphere and the period of fading is longer than for interference or polarization fading. A SID (Sudden Ionospheric Disturbance) is and extreme case of absorption fading. Skip fading Due to the changing heights of the layers the skip distance will be varying in accordance with the changing levels. If the Receiver is just as the edge of a skip zone the received signal can then rise and fall according to the skip distance. The effect of this type of fading is a very deep and very abrupt fading to a point where the signal will disappear and just as suddenly return to a good signal level. 6b.5 Recall that the highest frequency that will be refracted back to the transmitter is known as the Critical Frequency of Vertical Incidence (critical frequency).
If a range of frequencies from say 1MHz to 144MHz are transmitted towards the ionosphere then a frequency will be transmitted where no higher transmitted frequency signal is reflected to be heard on earth. This maximum frequency is called the Critical Frequency of Vertical Incidence or just Critical frequency. Recall that the highest frequency that will be refracted over a given path is known as the 'maximum usable frequency' (MUF) and that this will be higher than the critical frequency.
The Maximum usable frequency (MUF) is just what is says it is. It is the maximum frequency that reliable communication can take place over a know given path. The MUF is in fact a higher frequency the the Critical Frequency due to the fact that the critical frequency is a measure of what can be reflected when it meets the ionosphere at right angle to the layers where as the MUF is meeting the ionosphere at a lower angle. Thus higher frequencies than the Critical Frequency will continue to pass into outer space until the angle of at which the signal meets the ionosphere is low enough to reflect the signal. Similarly if there is a known path then the frequency can be raised higher than the Critical Frequency until it is not received at which point the MUF has been reached. Recall, in general terms how the MUF varies over the 24 hour cycle and the variation in MUF from summer to winter.
The diagram is a most simplified version of what happens to MUF, which are approximate only, over a 24 hour period without consideration of any special sun activity which can lead to enhanced propagation. The pattern to understand is that the MUF remain low until the sun rises and then there is a relatively steep increase in MUF until midday the highest point of the sun and then the decline in MUF is at a slower rate than in increase until sun set and then the decrease accelerates again until it reaches it lowest level about midnight. The peak levels of MUF are between 10:00 and 16:00 hrs Summer and Winter but in the Summer the MUF will reach as higher frequencies than in the Winter as the sun's rays are weaker in winter. 6b.6 Recall that the D layer tends to absorb the lower radio frequencies during daylight hours and that it tends to disappear at night. Understand that if the D-layer absorption occurs at frequencies higher than the MUF, then no ionospheric propagation can occur. The D layer is the layer that absorbs the lower radio frequencies and is a layer that disappears at night. The 1.8MHz band is particularly adversely affected by absorption in day light hours by the D layer and thus this band is of primary use at night. The 3.5MHz band is also adversely affected but not to the extent of the 1.8MHz band with daylight range of signals limited to about 250 miles where as during the night signals can reach half way round the world. it must be noted that if the D layer is absorbing frequencies lower than the MUF then no ionospheric propagation can occur as the signals cannot reach the upper layers. 6b.7 Recall which amateur bands will be "open" to support ionospheric propagation at different times of the day and year. Questions will be asked on 3.5 and 21MHz propagation over the 24 hour cycle. The 3.5MHz band is the night time band for propagation as during the day distance is limited to about 250 miles. The 21MHZ band and can be considered under normal propagation as the daylight band as it opens shortly after day break and close shortly after sun set. So these two band effectively mirror each other for propagation purposes.
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