High frequency (HF) waves between 3MHz and 30MHz are effectively reflected by ionized layers in the ionosphere producing the sky wave. Medium frequency waves may also be reflected, but less reliably.

The ionosphere contains several layers of ionized air at varying altitudes (Figure 1.12 ). The heights and density of the layers vary diurnally, seasonally and with the incidence of sunspot activity. The E and F2 layers are semi permanent while the F1 layer is normally only present during daytime.

Radio waves radiated at a high angle and reflected by these layers return to earth at a distance from the transmitter. The HF reflection process is in reality one of refraction in layers possessing a greater free electron density than at heights above or below them.

The speed of propagation is slowed on entering a layer and the wave is bent and, if of a suitable frequency and angle of incidence, returned to earth (Figure 1.13 ).

The terms used are defined as follows:
• Virtual height. The height at which a true reflection of the incident wave would have occurred (Figure 1.13).

• Critical frequency (fc). The highest frequency that would be returned to earth in a wave directed vertically at the layer.

• Maximum usable frequency (muf). The highest frequency that will be returned to earth for a given angle of incidence. If the angle of incidence to the normal is θ, the muf = fc/ cos θ.

• Skip distance. The minimum distance from the transmitter, along the surface of the earth, at which a wave above the critical frequency will be returned to earth (Figure 1.12 ). Depending on the frequency, the ground wave will exist at a short distance from the transmitter.

• Sporadic E-layer reflections. Reflections from the E layer at frequencies higher than those which would normally be returned to earth. They appear to be reflections from electron clouds having sharp boundaries and drifting through the layer. As the name implies the reflections are irregular but occur mostly in summer and at night.

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