The magnetic field and electric field oscillations perpendicular to each other or in the direction same as that of the wave propagation as basically known as Electromagnetic waves. The nature of electromagnetic waves is traverse. The speed of propagation of the electromagnetic waves is same as that of velocity of light. The scattering of the electromagnetic waves in space is uniform throughout. Suppose from a source O electromagnetic waves are generating, then in all the directions they will scatter with same strength. The propagation of the electromagnetic waves depends upon their properties.
Different types of propagation depending upon the properties are:
1. Sky wave propagation
2. Ground wave propagation
3. Space wave propagation
Ground wave propagation: The propagation of the waves along the surface of the earth is called ground wave propagation. It is also known as surface wave propagation. These waves have the ability to propagate along the surface of earth. These waves bend round the obstacles which come in the path of these waves. Due to this rounding through the obstacles, the intensity of the electromagnetic waves will decrease rapidly. If the intensity will increase then the long wave propagation of these waves will not be possible. Frequency also affects the propagation of waves along the earth’s surface. As the frequency will increase, the propagation signal will become weaker. Ground wave propagation is possible only for those waves which have very less frequency, nearly 2MHz. Due to such a less frequency range, it is also called medium wave propagation. Basically it is used for local broadcasting.
Their ranges are affected by some factors. Those are discussed below:
1. The frequency of radio wave.
2. The power of the transmitter used.
Sky wave propagation: The sky waves have frequency range between 2MHz to 30MHz. These radio waves have the ability to pass through earth’s atmosphere. The ionosphere of our earth reflects these rays very efficiently. When these rays move along the atmosphere then their movement is from transmitter towards the receiver antenna. This is called as sky wave propagation of the waves. In the above figure, b is the name assigned to the path of the sky wave propagation. These waves can have medium as well as high frequencies. So, due to this reason these waves can be used for long distance transmissions. The other name basically used instead of sky wave propagation is ionosphere propagation. This is called so because these waves after reflecting from the earth’s ionosphere, reach the receiver antenna. The distance covered after one reflection of sky wave from the ionosphere nearly approaches to 4000km. By using the technique of sky wave propagation the communication round the globe has become much easier.
Some important terms which are used more frequently in sky wave propagation are given below:
1. Maximum Usable frequency
2. Critical frequency
3. Skip distance
4. Fading
Critical frequency or CF: To study the Maximum usable frequency we have to first study the critical frequency.
Critical frequency of sky wave is that maximum frequency in which the sky wave after reflecting from the ionosphere return back to earth. If the frequency of the sky wave is increased then the critical frequency, it will not come back to earth after reflection.
Maximum usable frequency or MUF: In this the sky waves with maximum frequencies are sent at some angles towards the ionosphere. Then these rays will again be reflected by the ionosphere to earth.
This is the angle which is formed along the direction of the incident wave and the normal.
Skip Distance: When the sky wave is reflected back from the ionosphere having a constant frequency, but more than that of the critical frequency. Then the smallest distance from the transmitter to the earth’s surface covered by the sky wave is known as skip distance.
The distance of the sky wave from the transmitter to earth will increase if the angle made by the sky wave during reflection from the ionosphere surface will increase. So, in other words we can say that with the increase in angle the skip distance will also increase.
Fading: Due to the disturbance created by the waves, some variations can be seen in the signal strength at the receiver end. This is known as fading effect.
If the frequencies of the sky waves will be high, then the fading effect will increase. Errors in data transmissions and data retrievals are also caused by fading. Fading basically varies with time.
