| ADVANCED LESSON 55 | |
| LEARNING OBJECTIVES and NOTES | |
| Propagation -Ionosphere 2 | |
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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). 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. Recall, in general terms how the MUF varies over the 24 hour cycle and the variation in MUF from summer to winter. If you listen on a short wave receiver you may sometimes hear a signal sweep across the signal your are listening on. Some of these are ionospheric sounders. Basically a signal is transmitter with an antenna directing up to the ionosphere and a receiver listening to see if the signal is reflected. On every sweep there will come a frequency above which there is no reflected signal received. The highest signal to be reflected back is known as the Critical Frequency of Vertical Incidence (Critical frequency). Knowing that radio waves travel at the speed of light, measuring the time taken from the transmitter to the receiver and dividing by 2 gives the height of the reflective layer. If we change the angle at which we fire our signal upwards away from 90 degrees we are able to communicate at frequencies above the critical frequency. The highest frequency that will be refracted over a given path is known as Maximum Usable frequency (MUF) which is above the critical frequency. The MUF is constantly changing over a 24 hour period. The MUF will also change from summer to winter. If we send a signal towards the ionosphere horizontally rather than vertically we get an MUF of about 2.1 x critical frequency. This will vary depending on the direction we point the signal. From the UK the highest MUF tends to be towards S. America. |
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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. Lower radio Frequencies such as those in the 160 and 80m bands tend to be absorbed by the D layer during daylight. This is why we only tend to hear groundwave stations on 160m. during the day. When it becomes dark the D layer disappears and signals can pass through to the night time F layer which will reflect the signals. This is why it is possible to hear longer distance stations at night time. If D layer absorption takes place at frequencies higher than the MUF, then no ionospheric propagation can take place. For example if the MUF is at 3.7MHz and the D layer absorption takes place up to 4.0MHz then no reflection will take place on the 80m band because although the 3.5 to 3.7MHz signals should be reflected back to earth because they are below the MUF, the D layer absorbs all signals below 4.0MHz. |
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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. On 3.5MHz band during the summer the D layer will absorb radio waves and reduce the number of stations heard during the day. At night time, the D layer disappears and so 80M signals can be reflected by the night time F layer allowing signals to be heard half way round the world. On 3.5 MHz during the winter the D layer will disappear for more of the day, because the amount of light is less. So, propagation of signals will occur for longer in a morning and afternoon. During the night F layer propagation will be possible because the D layer is not present to absorb signals and so signals reach the night time F layers which do reflect signals. on 21MHz This is a prime band for DX contacts during peaks in the 11 year solar cycle when F2 propagation will extend well into the night. During 11 year minimal periods the F2 layer reflects during the day, but closes down at night. At the bottom of 11 year sun spot cycle there may be no F2 reflection during the day. In early summer and mid winter sporadic E may be observed, but signals are not as good on 24 and 28 MHz. |
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| Ground wave | |
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6c.1 Recall that the ground wave has a limited range due to absorption
of energy in the ground and that the loss increases with increasing
frequency. We have seen that the ground wave is the signal that travels along the ground rather than being reflected from layers of the ionosphere. This is the main path used by MW broadcast stations which by increasing the power during the day can have ground wave paths up to 100km. Ground waves are subject to high absorption by the earth and so there are limits to how far the signal will travel. The higher the frequency the higher the ground wave loss. For example at 3.0MHz attenuation of ground waves is 20 to 60 dB more than on 0.5MHz. So ground waves on 160m will travel further than on 10m. For many years vlf (3-30kHz) frequencies have been used to provide world wide communication with submarines via ground wave which is often across the sea! |
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