Sky waves are the radiowaves of frequency between 2MHz to 30MHz. These waves propagate from transmitter antenna to receiver antenna through sky and are reflected back by the ionosphere due to total internal reflection of the Earth's atmosphere. Therefore, their propagation is known as sky wave propagation. These waves are very useful for the long distance radio communication. Critical frequency It is the highest frequency of the radiowaves which can be reflected from ionosphere and returns to the Earth. If the frequency of the radiowaves is more than critical frequency, it will not be reflected back by ionosphere. Skip distance It is the smallest distance from a transmitter along the Earth's surface to the point, where the sky wave is first received after returning to the Earth. Fading When sky waves are received at receiver, interference takes place. The variation in the strength of a signal at receiver due to interference of waves is called fading.
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