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The skip distance of HF-transmission will increase with...
  • A
    lower frequency and higher level of the refracting ionospheric layer.
  • B
    higher frequency and lower level of the refracting ionospheric layer.
  • C
    higher frequency and higher level of the refracting ionospheric layer.
  • D
    lower frequency and lower level of the refracting ionospheric layer.

Refer to figure.


Radio waves can propagate in many different ways. The most common is as a "space wave". This is what most people think of when it comes to radio transmission, as this is line of sight, meaning that the radio waves travel in straight lines only.

Another propagation path is via ground waves, which "cling" to the surface and follow the contours of the Earth.

The third method of propagation is via sky waves, which are space waves that interact with the charged particles in the ionosphere. When these waves reach the ionosphere, the subsequent ionospheric attenuation refracts the waves, changing their course, until the waves are heading downwards again. This is often referred to as "reflection" by EASA, so do not be confused; it is refraction that occurs, it just looks like the reflection from afar, and they use the terms almost interchangeably.

The "skip distance" is the distance away from the transmitter where the first sky wave returns to the surface after bouncing off the ionosphere. This can create a dead zone of reception for some frequencies which travel as both ground and sky waves, as, for example the ground waves might stop at 100 nm, but the sky waves only begin at 200 nm (these numbers are not even close to correct).

Higher frequencies receive less ionospheric attenuation, therefore refracting less, and not bending as sharply in the ionosphere. This means that higher frequency radio signals do not "bounce" off the ionosphere at steep angles, thus making the skip distance longer. See the second annex above.

The height of the ionosphere also makes a difference, a higher ionosphere means that the signals will travel further before reaching the level where refraction begins, and will travel further on the way back down also. This means that a higher ionosphere contributes to a longer skip distance.

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