AIM OF THE SUBJECT WILL BE
COVERED IN THE FOLLOWING
PHASES :PHASE-I
:-INTRODUCTION
PHASE-II :-IMPORTANT TERMINOLOGY USED
PHASE-III :-PROPAGATION OF
ELECTROMAGNETIC WAVE
PHASE-IV :-SKY WAVE PROPAGATION
PHASE-V :-SPACE WAVE PROPAGATION
PHASE-VI :- GROUND WAVE PROPAGATION
PHASE VII:- SUMP UP
PHASE -I
Electromagnetic radiation is energy, which is
sent through free space or through a material
medium in the form of electromagnetic waves. An
electromagnetic wave is a transverse wave in
that the electric field and the magnetic field are
perpendicular to each other as well as to the
direction of the wave movement, at any point or
time in the wave. However, electromagnetic
waves are not affected by normal electric or
magnetic fields.
Electromagnetic waves have
properties similar to other forms of wave such as
reflection,
refraction,
diffraction
and
interference.
Electromagnetic waves were first
postulated by James Clark Maxwell and
subsequently confirmed by Heinrich
Hertz.
Electromagnetic waves - can be
imagined
as
a
self-propagating
transverse oscillating wave of electric
and magnetic fields. This diagram shows
a
plane
linearly
polarized
wave
propagating from right to left. The
electric field is in a vertical plane, the
magnetic field in a horizontal plane.
PHASE II
IMPORTANT TERMINOLOGY USED –
FREQUENCY:The frequency of a wave is its rate of
oscillation and is measured in HERTZ, the
unit of frequency, where one hertz is equal
to one oscillation per second.
RELATION:- Velocity = Frequency X
Wavelength
WAVELENGTH :A wave consists of successive troughs
and crests, and the distance between two
adjacent crests or troughs is called the
wave length
REFLECTION - When light is incident
upon some surfaces light can be reflected
from it
The direction that the light reflects is so that
it comes off the surface at the same angle as
shown in the diagram.
This is known as the law of reflection.
i.e. Angle of Incidence = Angle of Reflection
Note that the angles are measured from a line
that is at right angles to the surface. This is
known as the Normal.
REFRACTION When light is able to pass
through some materials such as glass or
Perspex a process known as refraction occurs.In
this process the light is changing medium. This
results in the wave slowing down when moving
to a medium of higher refractive
index (kind of like a material with higher
density) Devices that have glass lenses have
been developed to make use of the principle of
refraction. Glass can be used to bend the light
through them to magnify images in the case of
telescopes or microscopes and can also be used
in eye glasses to correct problems
Refraction
Diffraction is when a wave goes through a
small hole and has a flared out geometric
shadow of the slit. Diffraction is a characteristic
of waves of all types. We can hear around a
corner because of the diffraction of sound
waves. For instance, if a wall is next to you
when you yell, the sound will parallel the wall.
The wall may stop, but the voice doesn't; sound
will almost turn the corner of the wall. This is
diffraction.
Electromagnetic interference (or EMI, also
called radio frequency
interference or RFI . when in high
frequency or radio frequency) is
disturbance that affects an electrical circuit
due to either electromagnetic induction
or electromagnetic radiation emitted from
an external source.EMI can be
intentionally used for radio jamming as in
some forms of electronic warfare , or can
occur unintentionally.
POLORIZATION
:-
The physical orientation of electrical
field with respect to the direction of
propagation in free space is known as
polarization.
The act or process of propagating,
especially the process by which a
disturbance, such as the motion of
electromagnetic or sound waves, is
transmitted through a medium such
as air or water.
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.
SKIP DISTANCE:- Skip distance is the
shortest distance from a transmitter
measured along with the surface of the
earth at which a sky wave of fixed
frequency will be returned to earth.
SKIP ZONE
The skip zone is the area in which no
usable radio signal can be received
because it is shorter than the skip
distance but longer than the ground wave
range
MAXIMUM USABLE FREQUENCY:It is defined as the highest frequency that
can be used for sky wave communication
between the two points on the earth. It follows
that there are different value of M U F for
different pair of stations on the globe.
Radio frequencies and their primary mode of propagation
Band
VLF
LF
Very Low Frequency
Low Frequency
MF
Medium Frequency
HF
High Frequency (Short
Wave)
Frequency
Wavelength
Propagation via
3–30 kHz
100–10 km
Guided between the earth
and the ionosphere.
10–1 km
Guided between the earth
and the D layer of the
ionosphere.
Surface waves.
1000–100 m
Surface waves.
E, F layer ionospheric
refraction at night, when D
layer absorption weakens.
100–10 m
E layer ionospheric
refraction.
F1, F2 layer ionospheric
refraction.
30–300kHz
300–3000kHz
3–30 MHz
VHF
Very High Frequency
30–300MHz
10–1 m
Infrequent E ionospheric
refraction. Extremely rare
F1,F2 layer ionospheric
refraction during high
sunspot activity up to
80 MHz. Generally direct
wave.
Sometimestropospheric
ducting.
UHF
Ultra High Frequency
300–3000MHz
100–10 cm
Direct wave.
Sometimes tropospheric
ducting.
SHF
Super High Frequency
3–30 GHz
10–1 cm
Direct wave.
EHF
Extremely High Frequency
30–300GHz
10–1 mm
Direct wave limited by
absorption.
Propagation of an Electromagnetic Wave
Electromagnetic waves are waves which can travel
through the vacuum of outer space. Mechanical
waves, unlike electromagnetic waves, require the
presence of a material medium in order to transport
their energy from one location to another. Sound
waves are examples of mechanical waves while light
waves are examples of electromagnetic waves.
Electromagnetic waves are created by the
vibration of an electric charge. This vibration creates
a wave which has both an electric and a magnetic
component.
The mechanism of energy transport through a
medium involves the absorption and reemission of
the wave energy by the atoms of the material
How electromagnetic waves propagate in
Atmosphere?
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..
Different types of propagation depending upon
the properties are:
1. Sky wave propagation
2. Space wave propagation
3. Ground wave propagation
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.
LAYERS OF IONOSPHERE :-The ionosphere is
constituted of the following layers D ,E F1,F2 . Layer
F1 and F2 combines together during night and makes
a single layer .
D LAYER :-The D layer is the lowest existing at an
average height of 70 KM with an average thickness of
10 KM. The degree of its ionization depends on the
altitude of the sun above the horizon and thus
disappears during night. Least important from the
view of HF propagation.
E LAYER :- The E layer is next in height existing at
about 100 km with the thickness of 25 kms. Like D
layer it all but disappears at night. The reason of
disappearance is the recombination of the ions into
molecules . This is due to the absence of the sun at
night . The main effect of E layer are reflect some HF
F1 LAYER:F1 Layer exists at a height of 180 KM in day time and
combines with F2 Layer at night . Its Day time thickness is
about 25 Kms. Although some HF waves are reflected from
it , most passes to be reflected from F2 Layer.
F2 LAYER :The F2 layer is the most reflecting medium for high
frequency waves. Its approx thickness can be up to 200 Kms
and its height ranges from 250 kms to 400 kms in day time .
During night it fails to a height of 300 kms where it
combines with F1 Layer. It is most noticeable that F layer as
whole remains present in night also and most useful layer
for HF communication.
Space wave propagation -The radio
waves having high frequencies are basically called
as space waves. These waves have the ability to
propagate through atmosphere, from transmitter
antenna to receiver antenna. These waves can travel
directly or can travel after reflecting from earth’s
surface to the troposphere surface of earth. So, it is
also called as Tropospherical Propagation.
There are some limitations of space wave
propagation.
1. These waves are limited to the curvature of
the earth.
2. These waves have line of sight
propagation, means their propagation is
along the line of sight distance.
Ground wave propagation:
Ground wave propagation is particularly
important on the LF and MF portion of the
radio spectrum.
Ground wave radio propagation is used to
provide relatively local radio communications
coverage, especially by radio broadcast
stations that require to cover a particular locality.
Ground wave radio signal propagation is
ideal for relatively short distance propagation
on these frequencies during the daytime.
Ground wave or surface wave propagation
in which part of wave travels along or near
the surface of the earth. In ground wave
propagation the transmitting and receiving
Antenna should be closed to the surface
of the earth and the antenna should be
vertically polarized
Sky-wave ionosphere propagation is not
possible during the day because of the
attenuation of the signals on these frequencies
caused by the D region in the ionosphere.
In view of this, radio communications
stations need to rely on the ground-wave
propagation to achieve their coverage
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