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Basics of propagation: Definition and General Classification
Different Modes of propagation
Structure of ionosphere
Refraction and Reflection of sky wave by ionosphere.
Structure of Ionosphere:
In radio communication, skywave or skip refers to the propagation of radio waves reflected or refracted back toward Earth from the ionosphere, an electrically charged layer of the upper atmosphere.
The ionosphere is a region of the upper atmosphere, from about 80 km to 1000 km in altitude, where neutral air is ionized by solar photons and cosmic rays.
Ionization occurs due to the absorption of the ultraviolet and other high-energy radiation coming from the sun by air molecules.
As the wave enters into the upper part of the layer of DECREASING IONIZATION, the velocity of the radio wave decreases, and the wave is bent away from the Earth.
As the wave is in the highly dense centre portion of the layer, however, refraction occurs more slowly because the DENSITY OF IONIZATION IS ALMOST UNIFORM.
As a radio wave enters a region of INCREASING IONIZATION, the increase in velocity of the upper part of the wave causes it to be bent back towards the Earth.
The ionosphere is further subdivided into several layers, the details of which are given in Table.
Ionospheric Layers: C, D, E, F1, F2, Regions
Layers of high electrons densities : D, E, and F layers, as shown in Figure.
During the day the F layer splits into two layers called the F1 and F2 layers.
The D layer vanishes completely at night.
Refraction by the Ionosphere
When a radio wave is transmitted into an ionized layer, refraction, or bending of the wave, occurs.
Refraction is caused by an abrupt change in the velocity of the upper part of a radio wave as it strikes or enters a new medium.
The amount of refraction that occurs depends on three main factors:
the density of ionization of the layer,
the frequency of the radio wave, and
the angle at which the wave enters the layer
When high-frequency signals enter the ionosphere at a low angle they are bent back towards the earth by the ionized layer.
When operating at frequencies just below the MUF, losses can be quite small, so the radio signal may effectively "bounce" or "skip" between the earth and ionosphere two or more times.
If the ionization is not great enough, the wave only curves slightly downwards, and subsequently upwards as the ionization peak is passed so that it exits the top of the layer only slightly displaced. The wave then is lost in space.
To prevent this a lower frequency must be chosen.
Properties of Ionization
The degree of ionization varies with the height. The density of atmosphere decreases with height.
At great heights the solar radiation is intense but there are few molecules to be ionised.
The ionospheric layer acts as a reflector for a certain range of frequencies (3 to 30 MHz). EM waves of frequencies higher than 30 MHz penetrate the ionosphere and escape.
The phenomenon of bending of EM waves so that they are diverted towards the earth is similar to total internal reflection in optics.
Summary
Long distance communication between two points on the earth is
achieved through reflection of electromagnetic waves by ionosphere.
Sky wave propagation takes place up to frequency of about 30 MHz. Above this frequency, EM waves essentially propagate as space waves.
The reflections from the ionosphere are actually produced by refraction as the wave propagates through the ionosphere.
The ionosphere is a concentrated region highly charged ions and electrons that collective form an ionized gas or plasma.
The amount of refraction that occurs depends on three main factors:
the density of ionization of the layer
the frequency of the radio wave
the angle at which the wave enters the layer