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Ateneo de Naga University College of Engineering
ECEM416 Principles of Communications [Encoded Handout Edition VI] [@jjfm11] Page 1
RADIATION AND PROPAGATION OF WAVES
The travel of electromagnetic waves or sound waves through a medium, or the sudden electric
disturbance along a transmission line is called propagation of waves.
Such waves are either:
Light, radio, sound; heat, infrared, x-ray, ultraviolet ray, gamma radiation and
cosmic rays.
Before a wave can propagate it must be first radiated or is allowed to escape from a system.
In communication, radio wave is most
commonly employed.
Electromagnetic radiation comprises both an
Electric and a Magnetic field.
The two fields are at right-angles to each other
and the direction of propagation is right-angles
to both fields.
The Plane of the electric field defines the
Polarization of the wave.
ELECTROMAGNETIC WAVE
According to the theory Maxwell, an electromagnetic wave has the following important
characteristics:
1. It is composed of electric field and magnetic field, which are mutually perpendicular to each
other.
2. It is an oscillatory disturbance in free space that travels with a certain velocity. The light
velocity (
) in free space medium is the reference velocity.
3. It is a transverse wave (with oscillations perpendicular to the direction of propagation or
TEM wave.
4. It is a polarized wave. The basic polarizations are: Polarization refers to the physical
orientation in space of the electric field vector component of the electromagnetic wave.
Horizontal- electric field is parallel to the ground.
Vertical-electric field is perpendicular to the ground.
Elliptical-electric field rotates about the axis of the propagation direction but with
the electric and magnetic fields of unequal magnitudes.
Circular- electric field rotates about the axis of the propagation direction but the
electric and magnetic fields of equal magnitudes.
5. It is affected by the resulting medium to its attenuation or loss.
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Ateneo de Naga University College of Engineering
ECEM416 Principles of Communications [Encoded Handout Edition VI] [@jjfm11] Page 2
EFFECTS OF REAL ENVIRONMENT TO PROPAGATION OF RADIO WAVES (Propagation Mechanisms)
The actual propagation of waves is
normally happening closer to the ground
or along earths ground and not in free
space medium where the surrounding
environment has a direct of radio waves.
The following are important effects
attributed by the actual medium, the
earths atmosphere, to the radio wave
propagation.
Reflection of radio wave is the return or
change in direction of radio waves
striking or travelling from one medium to
another. It occurs at any boundary
between materials of differing dielectric
constants.
Refraction radio wave is the bending of
radio wave as it passes from one medium
to another in which the velocity of
propagation is different. It occurs when
the radio wave passes from one medium
to another of different density, the degree
of bending a wave at the boundary
increases with frequency.
Diffraction of radio wave is the bending of radio wave as
it passes the edges of an object or opening. It is caused
by interference between radio wave components
scattered by different parts of the object.
As a result of diffraction, electromagnetic waves can
appear to go around orners.
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Ateneo de Naga University College of Engineering
ECEM416 Principles of Communications [Encoded Handout Edition VI] [@jjfm11] Page 3
THE HUYGENS PRINCIPLE
A principle authored by Christian Huygens tried to
explain the existence of diffraction phenomenon of
electromagnetic waves.
A wave front is the line or curve of the crests
and troughs.
The direction of travel is perpendicular to the
wave front.
Each individual point is the center of its own
circular wave front.
A ording to his prin iple ea h point on wavelets.
PROPAGATION METHODS
Propagation methods refer to how a radio wave arrives
from a radio transmitting antenna into the receiving
antenna.
1. Ground wave method
2. Space wave method
3. Sky wave method
GROUND WAVE METHOD
The ground (surface) wave propagation method, the radio wave is radiated directly towards the
ground (earths ground is a good refle tor provided it is a good ondu tor ground).
The ground refle ts the radio wave towards the upper region of the earths atmosphere for it to be
refracted back.
It is used world-wide communication in the VLF and LF bands and for broadcasting in the MF
bands
Generally effective only up to 2MHz.
The ground wave signal should be vertically
polarized (vertical antennas are used) for it to
propagate along earths ground.
SPACE WAVE METHOD
Space wave (Tropospheric) Propagation
Method becomes compulsory when frequency generally exceeds 30MHz and beyond up to
300GHz.
It is used for sound and television broadcasting for radio relay systems, various mobile systems in
the VHF, UHF, and SHF bands.
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Ateneo de Naga University College of Engineering
ECEM416 Principles of Communications [Encoded Handout Edition VI] [@jjfm11] Page 4
It is limited by earths urvature (line-of-sight dependent) and heights of transmitting and
receiving antennas.
Follows two distinct paths from the transmitting antenna to the receiving antennaone through
the air directly to the receiving antenna, the other reflected form the ground to the receiving
antenna.
LIGHT-OF-SIGHT PROPAGATION
Signals in the VHF and higher range are not usually returned to earth by the ionosphere.
Most terrestrial communication at these frequencies uses direct radiation from the transmitter to
the receiver.
This type of propagation is referred to space-wave, line-of-sight, or tropospheric propagation.
SKY WAVE
The sky wave, often called the ionospheric wave, is radiated in an upward direction and returned
to earth at some distant location because of refraction from the ionosphere.
This form of propagation is relatively unaffe ted by the earths surfa e and an propagate
signals over great distances.
Generally effective above 2MHz up to 30MHz
Used for HF communication systems, including long-distance radio telephony and sound
broadcasting.
EARTHS ATMOSPHERE
With respect to radio wave propagation,
there are only three layers of the
atmosphere:
1. Troposphere
2. Stratosphere
3. Ionosphere
IONOSPHERE
Where is ionosphere?
It is from the upper limit of the stratosphere to a distance approximately 400 kilometers.
Beyond ionosphere is the outer space of free space and is ionized by solar radiation.
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Ateneo de Naga University College of Engineering
ECEM416 Principles of Communications [Encoded Handout Edition VI] [@jjfm11] Page 5
Ionization is the conversion of atoms or
molecules into an ion by light (heating or
charging) from the sun on the upper atmosphere.
Ionization also creates a horizontal set of stratum
(layer) where each has a peak density and a
definable width of profile that influences radio
propagation.
LAYERS OF THE IONOSPHERE:
1. D layer
2. E layer
3. F layer
D LAYER
D layer is the lowest layer of the ionosphere
existing at an average daytime height of 60
km and with an average daytime thickness
of 10km.
Degree ionization in the ionosphere
depends on the altitude of the sun
above the horizon.
This layer disappears at night due to recombination process.
The D layer is not an important layer for HF propagation for its main effect is to aid surface
wave propagation.
It can also refract back to earth to VLF and LF waves.
E LAYER
E layer is next to D layer in height. It is existing about 100km during the day and with a thickness
of roughly 25km.
Its main effect is to aid MF surface wave propagation but it can also reflect some HF waves in
daytime up to approximately 20MHz.
It exist at a height of 180km in daytime and combines with layer at night, its daytime thickness
is about 20km.
Although some of HF waves are reflected from it, most pass through to be reflected form the
layer. Thus the main effect of layer is to provide more absorption for HF waves.
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Ateneo de Naga University College of Engineering
ECEM416 Principles of Communications [Encoded Handout Edition VI] [@jjfm11] Page 6
LAYER
Layer is the most important layer of the ionosphere for refracting HF radio waves.
Its daytime thickness is approximately 200km with height that ranges from 250-400km during the
day. At night it combines with layer and falls to a height around .
FACTORS AFFECTING THE ABILITY OF INOSPHERE TO REFRACT RADIO WAVES
1. ION DENSITY
It is the refractive ability of the ionosphere increases
with the degree of ionization. The bending of a wave
at any given frequency or wavelength and angle of
radiation will increase with increase in ionization
density. The degree of ionization is greater in
summer than in winter and is also greater during the
day than night.
2. FREQUENCY OF THE RADIO WAVE
It is the bending of the wave at any given density and
angle of radiation will increase with a decrease in
frequency. The lower frequency, the more easily the
signal is refracted. The higher the frequency the more
difficult is the refracting or bending process.
3. ANGLE OF RADIATION (AOR) OR ANGLE OF TRANSMISSION
(AOT)
It is the bending of wave at any given ionization
density and angle of radiation (that is, the wave is
farther from the horizon).
SKY WAVE PROPAGATION PARAMETERS
Maximum Useable Frequency (MUF)
It is the highest frequency wherein the signal is able to return back to earth when beamed
at a certain angle.
f f se
f f
os
where f MU , f riti al fre uen y and angle of in iden e
Ranges from8MHz-35MHz, but may increase to 50MHz.
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Ateneo de Naga University College of Engineering
ECEM416 Principles of Communications [Encoded Handout Edition VI] [@jjfm11] Page 7
Lowest Useable Frequency (MUF)
It is the lower limit of the range frequencies that provide useful communications.
Frequency nearest the point where reception becomes unusable would be the LUF.
Skips distance
Minimum distance over which
communication at a given frequency
can be established by means of sky
wave.
Skip zone
Area where no signal can be heard.
Hop
Refers to a single reflection of a radio
wave from the ionosphere back to
earth.
Critical Angle ( )
It is the highest angle of radiation that
will return the wave of earth at a given
density of ionization in the layer for
frequency.
OTHER PROPAGATION MODES: SCATTER PROPAGATION METHODS
Tropospheric Scatter
It make use of the scattering of radio waves in the
troposphere to propagate signals in the 250MHz-
5GHz range
Ionoscatter
It is much similar to tropospheric scatter, except
that scattering medium is the E region of the
ionosphere, with some assistance of D and F layer.
FACTORS AFFECTING OPTIUM OPERATING FREQUENCY
Location and Geography
Ionizing radiation varies with locations and altitudes.
Seasonal Variations
Variations brought by the revolution of the earth around the sun.
Diurnal Variations
Variations brought by the rotation of the earth around its axis.
Cyclical Variation
Variations brought by the solar cycle like the sunspot activities (e.g. 11 years sun spot cycle
and 27 days cycle).
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Ateneo de Naga University College of Engineering
ECEM416 Principles of Communications [Encoded Handout Edition VI] [@jjfm11] Page 8
IRREGURALITIES OF SPACE WAVE PROPAGATION
Superrefraction or Ducting occurs when the refractive index of the air decreases with height much
more rapidly than normal.
Duct is a region in which superrefraction occurs.
Subrefraction reduces signal strength by bending the rat away from the receiving point.
FADING
Fading is the variation in signal amplitude at the receiver caused by changes in the characteristics
of the signal path.
Fading causes the received signal to vary in amplitude, typically making it smaller.
Fading is caused by three factors:
1. Variation in distance (due to relative motion) between transmitter and receiver.
2. Changes in the environmental characteristics of the signal path (e.g. an obstruction gets
in the way).
3. The presence of multiple signal paths.
Most significant types of fading:
Shadow fading
It is caused by object coming between the transmitter and receiver.
Multipath fading
The transmitted signal takes multiple paths to the receiver due to reflections (off
buildings, water towers, hills and mountains, moving vehicles); these signals can
add to or subtract from each other at the receiver.
Fading is most pronounced at UHF and microwave frequencieswavelengths are short compared
to path distances and size of reflective surfaces.
Selective fading
It occurs at shortwave frequencies (3-30MHz) caused by relatively rapid variations
in the refraction in the ionosphere.
DIVERSITY
A diversity system uses multiple transmitters, receivers, or antennas to mitigate the problems
cause by multipath signals.
With frequency diversity, two separate sets of transmitters and receivers operating on
different frequencies are used to transmit the same information simultaneously.
Space or spatial diversity uses two antennas spaced as far apart as possible at the receiving
site.
In both types, multiple signals are received and compared or combined to determine the message.
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