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| | ==Additional text== | | ==Additional text== |
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| - | As the frequency increases, each wave is refracted less by the ionisation in the layer, and so each penetrates further before it is reflected. As a wave approaches the reflection point, its group velocity approaches zero and this increases the time-of-flight of the signal. Eventually, a frequency is reached that enables the wave to penetrate the layer without being reflected. For ordinary mode waves, this occurs when the transmitted frequency (fo) just exceeds the peak plasma frequency of the layer. These frequencies are identified by the layer where reflection takes place (foE, foF1, foF2 and foEs). In the case of the extraordinary wave, the magnetic field of the earth enhances the reflection capability of the ionosphere and reflection occurs at a frequency (fx) that is higher than the ordinary wave by half the electron gyrofrequency.
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| - | The frequency Fc at which a wave just penetrates a layer of ionisation is known as the critical frequency of that layer. The critical frequency is related to the electron density of the specific layer (D, E, F1 or F2).
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| - | All transmitted frequencies above this critical frequency will penetrate the layer without being reflected. Their group velocity will however, will be slowed by any ionisation, and this will add to the time-of-flight. If such a wave encounters another layer, whose plasma frequency is higher than the frequency of the wave, it will be reflected, and the return signal will be further delayed as it travels back through the underlying ionisation. Therefore, HF frequencies between 5 and 30 MHz pass through the E layer and are reflected at the F layer. Similarly, MF frequencies (AM broadcast stations) at night pass through the D layer and are reflected by the E layer.
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