Multiple Hops & Reflections with Ionospheric Propagation
When using ionospheric propahation for global radio communications, it is likely the signal will undergo several hops. This may not always occur as you think.
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The maximum distance that can be achieved when using a reflection from the F2 region is around 4000 km and for the other reflective regions in the ionosphere it is somewhat less By simply using a reflection from the ionosphere, this does not explain how signals travel from the other side of the globe using ionospheric propagation. The reason this is possible is that signals undergo several reflections. It is found that after the signals return to Earth from the ionosphere they are reflected by the Earth's surface and returned back up to the ionosphere. Here they are reflected again by the ionosphere, being returned to the Earth a second time about twice the distance away from the transmitter that a single reflection would give.
Unfortunately the signal undergoes additional attenuation as might be expected. Each reflection by the Earth introduces some losses and therefore the signals are attenuated each time they are reflected. The surface of the Earth at the point of reflection has a major effect on the level of these losses. Sea water is a very good reflector as might be expected, but dry desert is very poor. This means that signals that are reflected in the Atlantic are likely to be stronger than those reflected by a desert region.
Apart from the reflection at the Earth's surface the signal suffers losses in the ionosphere as well. Every time the signal passes through the D region there is an additional amount of attenuation. This can be very important because the signal has to pass through the D layer twice each time it is reflected by one of the higher regions and with more than one hop, the signal passes through the D region several times. As already mentioned the attenuation reduces with frequency. Apart from the fact that high frequency paths are more likely to use the F2 layer and have less reflections, the high frequency path will also suffer less loss from the D layer. This will mean that a signal on 28 MHz, for example, will be stronger than one on 14 MHz assuming that propagation can be supported at both frequencies.
It should also be remembered that the path length for a multiple reflection signal will be greater than the great circle distance around the globe, especially if high angles of radiation are used. This in itself will add to the signal loss because the loss is proportional to the path length.
At some times a tilt in the ionospheric regions and in particular the F2 region may occur. When this happens the signal may not be reflected back to Earth. Instead it is reflected so that it travels between the Earth and the ionosphere, before meeting the ionosphere again where it can be reflected back to earth. As this form of propagation does not involve a reflection from the surface of the earth, the losses are much lower and accordingly signal strengths provided are higher. In some research it has been proposed that this form of propagation could account for round the world echoes.
The tilt or distortion in the ionosphere required to produce this form of propagation known as Chordal Hop occurs near sunrise and sunset and across the equator. The propagation using the equatorial anomaly generally occurs in a north-south (or south-north direction). It is found that the F2 region is higher across the equator and this means that either side of this it tilts, enabling the signals to be reflected above the earth's surface.
There are a variety of mechanisms by which signals can propagate. With propagation over long distances the path may be a complicated summary of several hops utilising various regions. As the state of the ionosphere is not constant around the globe, reflections may use the F region in one area, but the E region for signals on the same frequency in another. In some instances signals may even become trapped between the F and E regions where there is a valley in the level of ionisation, and interestingly it is found that MUFs for this mode of propagation are higher than they are for the equivalent path using a double hop.
To be able to categorise the various paths that can be taken a form of nomenclature has been devised. Shown in Figure 5.2, it indicates the regions where the reflection takes place and a ground reflection is indicated by a hyphen between two letters. Thus F would indicate a single hop using the F region whereas E-F would indicate a reflection by the E region returning the signal to earth where it is reflected back up to the F region before returning to Earth at the receiver.
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