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Sky Wave Propagation

What is radio wave propagation?

To understand the meaning of skywave propagation, defining radio wave propagation is important. Radio wave propagation is the behavior of radio waves as they propagate from one point to another or into various parts of the atmosphere.

Following is the classification of these waves:

  • Ground waves propagation
  • Skywave propagation
  • Free space propagation

What is sky wave propagation?

Skywave propagation also known as the skip is a type of radio wave propagation. It is either the reflected or refracted back waves to the earth from the ionosphere which is an electrically charged layer of the upper atmosphere.

Medium and shortwave frequencies can be refracted back to earth which is beyond the horizon which makes them useful in the transcontinental transmission of the waves. Following is the table explaining the distribution of frequencies MUF (maximum usable frequency) and OWF (optimum working frequency) depending upon the layers of the earth:

Layers D E F1
MUF (MHz) 16 28 16
OWF (MHz) 13.6 23.8 13.6

What is critical frequency?

Critical frequency is defined as the maximum frequency at which the total internal reflection takes place from the ionosphere. The mathematical representation is given as:

\(\begin{array}{l}f_{c}=9\sqrt{N_{max}}\end{array} \)


fc: critical frequency in Hz

Nmax: maximum electron density per m3

The mathematical representation of critical frequency as a function of MUF is:

\(\begin{array}{l}f_{c}=\frac{MUF}{sec\Theta }\end{array} \)


fc: critical frequency in Hz

MUF: maximum usable frequency

Ө: angle of incidence

Critical frequency varies depending upon atmospheric conditions, time of the day and the angle of fire of the radio waves by the antenna.

What is skip distance?

Skip distance is defined as the minimum distance from the earth’s surface and the point from where the radio signal is been transmitted. For a flat earth skip distance is given as:

\(\begin{array}{l}D_{SKIP}=2h\sqrt{(\frac{f_{MUF}}{f_{C}})^{2}-1}\end{array} \)


DSKIP: skip distance

h: height at which reflection happens

fMUF: maximum usable frequency

fc: critical frequency

Skywave propagation applications:

  • Satellite communications take place with the help of skywave propagation as it is dependent on the upper atmospheric conditions
  • Mobile communications

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