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Sky Wave
The ionization levels in the layers increase towards the centre of the layer. This means that as a radio wave transits a layer it encounters an increasing density of ions as it moves to the centre of the layer and decreasing density as it moves out of the layer. If the radio waves travel across the layer at right angles they will be retarded, but will maintain a straight path. However, if the waves penetrate the layer at an angle they will be refracted away from the normal as they enter, then back towards the normal as they exit the layer.
Radio Propagation Theory 2
Figure 2.9 Sky wave propagation - critical angle
The amount of refraction experienced by the radio waves is dependent on both the frequency and the levels of ionization. If the radio wave refracts to the (earth) horizontal before it reaches the centre of the layer then it will continue to refract and will return to the surface of the earth as sky wave; this is total internal refraction at the layer.
Starting from the vertical at the transmitter, with a frequency which penetrates the ionosphere, as the angle between the vertical and the radio wave increases, an angle will be reached where total internal refraction occurs and the wave returns to the surface. This is known as the first returning sky wave and the angle (measured from the vertical) at which this occurs is known as the critical angle. The distance from the transmitter to the point where the first returning sky wave appears at the surface is known as the skip distance. As sky waves occur in the LF, MF and HF frequency bands there will also be some surface wave present. From the point where the surface wave is totally attenuated to the point where the first returning sky wave appears there will be no detectable signal, this area is known as dead space.
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Figure 2.10 Sky wave propagation - dead space
The height at which full internal refraction occurs is dependent on frequency, but, as a generalization frequencies up to 2 MHz will be refracted at the E-layer and from 2 – 50 MHz at the F-layers. Sky wave is only likely to occur above 50 MHz when there are abnormal ionospheric conditions associated with intense sunspot or solar flare activity, therefore, VHF frequencies used for navigation systems do not produce sky waves.
Effect of Change in Ionization Intensity
Since the reason for the refraction is the ionization of the upper atmosphere it follows that if ionization intensity changes, then the amount of refraction of radio waves will also change. At a given frequency, as ionization increases the refractive index and hence the amount of refraction affecting the radio waves will also increase. This means that refraction will take place at a smaller critical angle and the skip distance and dead space will decrease. Conversely, a decrease in ionization will result in an increase in critical angle, skip distance and dead space.
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Figure 2.11 Sky wave propagation - effect of increased ionization |
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Effect of Change of Frequency
For a given ionization intensity, the amount of refraction of radio waves decreases as frequency increases, because as frequency increases the energy contained in the radio wave increases and therefore refraction decreases. So, as frequency increases, the critical angle will increase and the skip distance and dead space will also increase. As frequency increases, the surface wave range will decrease, so there is an increase in dead space caused by both the increase in skip distance and decrease in surface wave range. Conversely, a decrease in frequency will give a decrease in critical angle, skip distance and dead space.
Height of the Layers
The skip distance will also be affected by the altitude of the refracting layers. As the altitude of the layer increases then the skip distance will also increase and greater ranges will be experienced by refraction at the F-layer than the E-layer.
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LF and MF SkyWave Propagation
During the day the D-layer absorbs radio energy at frequencies below about 2 MHz (LF and MF bands). At night the D-layer is effectively non-existent so, at these frequencies, sky waves, refracted at the E-layer are present. This means the sky waves at LF and MF are not reliable for continuous long range use and the presence of sky waves at night at the relatively short ranges associated with these lower frequencies will cause interference with short range navigation (and broadcasting) systems relying on surface wave reception. This affects ADF and will be discussed in more detail in Chapter 7.
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Figure 2.12 LF/MF Sky wave propagation
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Achievable Ranges
The maximum range for sky wave will be achieved when the path of the radio wave is tangential at the surface of the earth at both the transmitter and receiver.
A simple calculation shows that the average maximum range for refraction from the E-layer at 125 km is 1350 NM, and the average maximum range from the F-layer at 225 km is 2200 NM. These ranges will obviously change as the height of the ionized layers changes.
Multi-hop sky wave occurs when the wave is refracted at the ionosphere then the sky wave is reflected back from the surface of the earth to the ionosphere etc. Multi-hop sky wave can achieve ranges of half the diameter of the earth.
Figure 2.13 Multi-hop Sky wave propagation
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