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.
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.
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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Team HiTech
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