Radio Wave Propagation Part 2: Refraction: How Sky Waves Penetrate the Ionosphere for Satellite Comm
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Sky wave is the most frequently used method for long distance transmission.
Sky waves are radiated in a direction towards the sky at a relatively large angle with reference to the earth. The radio waves are refracted back to earth by the ionosphere.
Sky waves are affected by changing conditions in the ionosphere as it depends on the ionosphere to refract the signal back to earth.
Sky waves propagation depends on:
Time of the day
Locations of transmitter and receiver
Sunspot activity (when sunspot activity are low, freq above 20 MHz are unusable because E & F layers are too weakly ionised to refract signals back to the earth)
Sudden ionospheric disturbances due to solar flares causing most HF signals to be absorbed at the D layer
Sky waves are used for HF communications.
The Ionosphere is divided into four distinct regions of
D Layer
E Layer
F1 Layer
F2 Layer
It ranges from 48 km up to 400 km.
These layers vary in location and in ionization level with the time of the day. They also fluctuate in a cyclical pattern throughout the year
The ionosphere refracts radio waves of specific frequencies, primarily HF (3 –30 MHz “shortwave”).
It is this refraction of radio energy that makes worldwide HF radio communication possible without the aid of satellites.
Each layer represent an increase in ion density on the one below it. Thus, the lowest layer, the D layer, is the weakest and disappear at night.
Refraction
The bending of the radio waves as it move from one medium into another medium due to the radio waves traveling with a different velocity of propagation in different medium.
Advantages of Sky wave
High frequency transmission offers the advantage of long-distance communication and has a higher survival rate in harsh environments.
The use of suitable High Frequency radio solved the communication problem in thick jungle, mountainous terrain and in environment where Line-Of- Sight transmission are often not possible.
Disadvantages of Sky wave
Satellite and terrestrial communications are vulnerable to physical destruction and electronics countermeasure attack during the war.
Quality and reliability of high frequency radio wave propagation is dependent on frequency, times of the day, seasons and degree of ionospheric disturbances.
In other words, one frequency that may be propagating well during certain periods of the day or night but may be poor or cannot be used during other periods of the day or night.
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