Sky Wave Propagation
The radio waves which travel from transmitting to the receiving antenna after being reflected from the ionosphere are called Sky waves and phenomenon is known as Sky wave propagation.
|Sky Wave Propagation|
This type of propagation is used for wave frequency above 1500 kHz. Infact, for radio communications of radio frequency above 1500 kHz only sky wave can be used.
The ionosphere is upper portion of the atmosphere, which absorbs large quantities of radiant energy from the sun, becoming heated and ionized. There are variations in the physical properties of the atmosphere, such as temperature, density and composition. Because of this and the different types of radiation received, the ionosphere tends to be stratified, rather than regular, in its distributions. The most important ionizing agents are ultraviolet and alpha, beta and gamma radiation from the sun, as well as cosmic rays and materials.
The ionosphere is generally considered to be divided into 3 layers, the D layer, the E layer and F layer; the F layer is subdivided into the F1 and F2 layers.
The D and E layers, the farthest from the sun, are weakly ionized. They exist only during daylight hours, during which they tend to absorb radio signals in the medium-frequency range from 300 kHz to 3 MHz.
The F1 and F2 layers, the closest to the sun, are the most highly ionized and have the greatest effect on radio signals. The F layers exist during both day and night.
The primary effect of the F layer is to cause refraction of radio signals when they cross the boundaries between layers of the ionosphere with different levels of ionization. When radio signals goes into the ionosphere, the different levels of ionization cause the radio waves to be gradually bent. The direction of bending depends on the angle at which the radio waves enters the ionosphere and the different degrees of ionization of the layers, as determined by snell's law.
At great heights, the solar radiation is intense but the density of atmospheric air is extremely low. As a result, the ionization is again low. However, at some intermediate heights, there occurs a peak of ionization density. The ionosphere layer act as a reflector for a certain range of frequency, ranging from 3 MHz to 30 MHz. The phenomenon of bending of electromagnetic waves, so that the waves are diverted back towards the Earth, is similar to the total internal reflection of light waves while passing through the air of continuously decreasing refractive index.
The electromagnetic waves of frequencies higher than 30 MHz penetrate the ionosphere, without being reflected back, and thus escape. Therefore, sky wave methods suits for propagation of electromagnetic waves ranging from few MHz to about 30 MHz only. The method is commonly used for standard AM short-wave radio broadcast.
Advantages of Sky Wave Propagation
- Sky wave propagation supports large distance propagation.
- The frequency range of this propagation is considerably high.
- In this propagation attenuation due to atmospheric conditions is less.
Disadvantages of Sky Wave Propagation
- Long-distance propagation requires large ionized antennas.
- Due to the presence of the ionosphere near and far during night and day respectively there exist variation in signal transmission in day and night.