Basics of Radio Wave Propagation || Electromagnetic Wave Propagation || SURFACE 🆚 SPACE 🆚 SKY WAVE Propagation

Electromagnetic or Radio Wave Propagation:

The electromagnetic radiated waves Travel through different paths to reach at the receiver antenna. The path taken by the wave to travel from the transmitter and to reach at the receiver antenna is known as wave propagation.

The propagation of electromagnetic waves may be divided into the following categories:

1.        Ground Wave or Surface Wave Propagation

2.        Sky Wave Propagation 

3.        Space Wave Propagation (Direct wave follow -Line of Sight propagation and indirect or reflected waves have two components:

a)         Reflected From Ground

b)         Reflected from Troposphere).

 

Ground Wave Propagation : A ground or surface wave travels along the surface of the earth. These waves are vertically polarized therefore a vertical antenna is useful  for these radio waves to transmit. If a horizontally polarized wave is propagated as a ground wave, due to conductivity of the earth the electric field component of the electromagnetic waves gets short circuited and will not travel for a significant distance from the transmitter.  Due to curvature of the earth vertically polarized radio waves become parallel to the earth after travelling some distance and get attenuated. Instead of just travelling in a straight line radio signal tends to follow the curvature of the Earth. This is because currents are induced on the surface and cause the Wave front of the radio waves to tilt down towards the earth and due to this tilted wavefront ground waves are able to curve around the earth and these signals can be received beyond the horizon. 

  

Since ground losses increase rapidly with the increase of frequency hence this propagation is used practically in the low frequency and medium frequency portion of the radio spectrum i.e. upto 3 MHz. The maximum range or coverage of the ground wave propagation not only depends upon the frequency but also depends  on the power of the transmitter.

 Sky Wave Propagation  :

 To understand Sky wave propagation, first we have to get an idea about IONOSPHERE. The ionized region in the upper part of the atmosphere of the earth is called IONOSPHERE.

Ionosphere starts from 48 kms to 965 kms. From the Earth surface. There are 3 significant layers of the ionosphere responsible for the sky wave propagation.














These Layers are :      D-Layer, (60 kms to 90 kms)

E- Layer (90 kms to 150 kms)

F-Layer  (150 kms to more than 500kms)

F layer is further divided into

F1- layer

F2-Layer

During night time only E and F layers exists and D layer merged to E layer. We have understood ionosphere now we will discuss Sky Wave Propagation.  In the sky wave propagation the upward transmitted electromagnetic waves reflect from the ionosphere and return back on the Earth surface.


            Every long Radio Communication of medium frequency and high frequency are conducted using Sky wave propagation. Short waves are mainly transmitted using this type of propagation.  This propagation is preferred for 3 MHz to 30 MHz of frequency band.  



Critical Frequency: Critical frequency for any layer of ionosphere determines the highest frequency that will be returned down to the earth by that layer, after having been beamed by the transmitter straight up into the sky. The maximum frequency that gets reflect back and reaches to the receiver antenna at earth surface is called critical frequency.

  

Space Wave Propagation:  Space Wave Propagation takes place when the radio waves from a transmitter propagates through space around the earth and reach at receiver. The radio waves can propagate either directly or after reflection from ground or from troposphere.

            This type of radio waves propagate within 20 kms from the earth surface i.e. reflected from troposphere. Such propagation comprises of two waves :

1.    Direct Wave :- Follow LOS –Line of Sight

2.    Indirect Wave :- It has two components:-

a)    Reflected Wave from Ground Surface

b)    Reflected Wave from Troposphere.

 

 

Line of Sight Propagation -LOS  In the Line of Sight propagation, as name implies the wave travels a minimum distance of sight, which means that wave travels in a line of sight from transmitter to receiver. LOS propagated waves travel in a direct path from the source i.e. from transmitter to receiver. The LOS propagation will not be smooth if there occurs any obstacle in its transmission path.

 

Frequencies above 30 MHz i.e. VHF & higher frequencies preferred to follow LOS propagation or Space wave propagation, because when these frequencies propagates at lower levels of atmosphere, neither diffraction nor reflection affects them significantly. Thus any obstruction between transmitting and receiving antenna will block the signal. The farthest possible point of propagation referred to as the radio horizon. The two stations which are not in line of sight may also be able to communicate through an intermediate radio repeater station.




Applications of Space Wave

1.    TV Broadcast

2.    Microwave links

3.    DTH service

4.    Satellite communications

5.    LOS communications for aviation and maritime use.

6.    Radar communication.

 

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Comments

  1. Very useful tutorial as well as documentation for all who exactly want to learn propagation.

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