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 behaviour 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?

Sky Wave propagation, commonly known as the skip, is a kind 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:

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

Where,

fc: critical frequency in Hz

Nmax: maximum electron density per m3

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

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

Where,

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 the skip distance?

Skip distance is the minimum distance between the earth’s surface and the radio signal’s transmission point. For flat earth, skip distance is given as:

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

Where,

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

Frequently Asked Questions – FAQs

Q1

What is the purpose of skywave?

Skywave broadcasts can be utilised for both very local communications (almost vertically directed waves) and long-distance communications (DX) (Near Vertical Incidence Skywaves – NVIS)
Q2

How does Skyway propagation work?

An example of radio wave propagation is skywave propagation, sometimes called the skip. The ionosphere, an electrically charged region of the high atmosphere, is responsible for returning waves to the ground that are either reflected or refracted.
Q3

Sky waves have electromagnetic properties?

Sky waves are electromagnetic waves transmitting antennae emit and then receive after being reflected by the ionosphere. This method of propagation is known as sky wave propagation. The ionosphere’s entire internal reflection of the electromagnetic waves causes the skywaves to propagate.
Q4

What are the main key points of Skywave propagation?

Skywaves, skip distance, and skip zone are three crucial concepts in ionospheric HF propagation and radio communications. It is possible to make the most of the impacts of ionospheric propagation by understanding how HF radio signals really travel across space.
Q5

Why does sky wave propagation differ at night and day?

The ionosphere’s composition changes at night compared to during the day because of the sun’s presence or absence.

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