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Radiation and
Propagation of Waves
Chapter 8
Standard Text Book
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Objectives
Understand wave, electromagnetic waves
Radiation and associated phenomenon
Explain Reflection
Refraction
Diffraction
Polarization
Describe the Propagation of waves Ground Waves
Sky waves
Space Waves
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Wave
Wave is a mode of transfer of energy
Transverse waves
Longitudinal waves
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Transverse Waves
Transverse waves are those whose directionof propagation is perpendicular to both the
electrical field and the magnetic field Theelectrical field and the magnetic fields lie inplanes that are perpendicular to each other.(x and y planes)
Thus the direction of propagation will be inthe z plane or third dimension
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Electromagnetic Waves
Consist of
Magnetic wave
Electrical wave Most of the energy is returned to the circuit.
If it isnt, then some it must be set free orradiated. Radiated energy is not desirable.
But if such power is escaped on purposethen it is said to be radiated
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Wave Propagation Example
electric
field
magnetic
field
propagation direction
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Electrical to Magnetic Conversion
The antennas are the transducers
The transmitting antenna changes the electrical
energy into electromagnetic or waves The receiving antenna changes the
electromagnetic energy back into electrical energy
These electromagnetic waves propagate at
rates ranging from 150kHz to 300GHz
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Radio-frequency Interference
If the radiated energy comes from anotherradio transmitter, then it is considered radio-
frequency interference (RFI) The transmitting antenna should be
specifically designed to prevent the energyfrom being returned to the circuit.
It is desirable that the antenna free theenergy in order that it might radiate intospace
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Electromagnetic Interference
If the energy comes from else where,then it is electromagnetic interference
(EMI)
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A few Concepts at a glance
Free Space
Which does not interfere with normal radiationand propagation
Point Source
A simple point acting like a source radiating in alldirections
Power density Power per unit area
Isotropic source
one which radiates uniformly in all directions
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Polarization of the Electrical Field
The polarization of the electrical field isdetermined by the direction of
oscillations If the oscillations are in the vertical
direction then the polarization is said to bevertical
If the oscillations are in the horizontaldirection then the polarization is said to behorizontal
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Polarization of antenna
Thus a vertical antenna will result in avertically polarized wave.
A vertical antenna is one that consistsof a vertical tower, wire, or rod, usuallya quarter wavelength in length that is
fed at the ground and uses the groundas a reflecting surface.
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Wavefronts
A wavefront is a plane joining all pointsof equal phase in a wave
Take a point in space. Imagine wavesradiating outward in all directions fromthis point. The result would resemble a
sphere. The point of radiation is calledthe isotropic point source
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Isotropic Power
Since the power at any point away from theisotropic point is inversely proportional to the
square of the distance from the point, thenthe power decreases rapidly the further awayfrom the point you need.
Although the wavefront is curved in shape,
from a distance small sections appear planarand can be thought of as plane wavefronts
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Reflection
Reflection is the abrupt reversal indirection
Caused by any conductive medium suchas Metal surfaces or
Earths surface
There will normally be a shift in phase
Coefficient of reflection is less than 1
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Complete Reflection
Complete reflection will occur only inperfect conductors and when the
electric field is perpendicular to thereflecting element or medium
Coefficient of Reflection will be 1
Coefficient of Reflection is the ratio ofthe reflected wave intensity to theincident wave intensity
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Refraction
Occurs when the waves pass from onemedium to another whose densities are
different Coefficient of reflection is less than 1
The angle of incidence and the angle of
refraction is related by Snells Law
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Refraction
Refraction (or bending) of signals is due to temperature, pressure,and water vapor content in the atmosphere.
Amount of refractivity depends on the height above ground. Refractivity is usually largest at low elevations.
The refractivity gradient (k-factor) usually causes microwave signalsto curve slightly downward toward the earth, making the radiohorizon father away than the visual horizon.
This can increase the microwave path by about 15%,
Normal
Refraction
Refraction (straight line)
Sub-Refraction
Earth
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Diffraction
Waves traveling in straight lines bend aroundobstacles
Based on Huygens principle (1690) Each point on a wavefront can be thought of as an
isotropic point or a source of secondary sphericalenergy
Concepts explains why radio waves can beheard behind tall mountains or buildings thatare normally considered to block line of sighttransmissions
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Attenuation and Obstructions
Longer the wavelength (lower frequency) of the
wireless signal, the less the signal is attenuated.
Same wavelength
(frequency), less
amplitude.
Shorter the wavelength (higher frequency) of the wireless
signal, the more the signal it is attenuated.
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Propagation..
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Ground and Space Waves
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T3-23
Ground-Wave Propagation
The curved surface of the Earth horizon can diffract long-wavelength
(low frequency) radio waves. The waves can follow the curvature of the
Earth for as much as several hundred miles.
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T3-24
Ground-Wave Propagation
Results from a radio wave diffraction along theEarths surface.
Primarily affects longer wavelength radio waves thathave vertical polarization (electric field is orientedvertically).
Most noticeable on AM broadcast band and the 160meter and 80 meter amateur bands.
Communication distances often extend to 120 milesor more.
Most useful during the day at 1.8 MHz and 3.5 MHzwhen the D-Region absorption makes sky-wave
propagation impossible.
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Attenuation related to frequency
Loses increase with increase in frequency
Not very effective at frequencies above2Mhz
Very reliable communication link
Reception is not affected by daily orseasonal weather changes
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Used to communicate with submarines
ELF (30 to 300 Hz) propagation is
utilized
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Sky WavePropagation
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T3-28
Atmospheric Regions
Region Height Notes
Troposphere 7 miles Region where all weather occurs
Stratosphere 6 to 30miles
Region where atmospheric gasesspread out horizontally. The high
speed jet stream travels in the
stratosphere.
Ionosphere 30 to 400miles
Region where solar radiation fromthe sun creates ions. Major
influence on HF radio wave
propagation.
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T3-29
Atmospheric Regions
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What is the ionosphere?
The ionosphereis the uppermost part ofthe atmosphere, distinguished because itis ionizedby solar radiation.
At heights above 80 km (50 miles), the
atmosphere is so thin that free electronscan exist for short periods of time beforethey are captured by nearby ions.
This part of the atmosphere is ionized andcontains a plasma.
In a plasma, negative free electrons andpositive ions are attached by theelectromagnetic force, but they are tooenergetic to stay fixed together in neutralmolecules.
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T3-31
Sky-wave Propagation
Ionization levels in the Earthsionosphere can refract (bend) radio
waves to return to the surface. Ions in the Earths upper atmosphere are
formed when ultraviolet (UV) radiation andother radiation from the sun knockselectrons from gas atoms.
The ionization regions in the Earthsionosphere is affected the sunspots on the
suns surface
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FIGURE 12-9 Sky-wave propagation.
Gary M. Miller, Jeffrey S. BeasleyModern Electronic Communication, 7e
Copyright 2002 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
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T3-33
Sky Wave Propagation
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Radio waves radiated from the transmittingantenna in a direction toward the ionosphere
Long distance transmissions
Sky wave strike the ionosphere, is refractedback to ground, strike the ground, reflectedback toward the ionosphere, etc until it
reaches the receiving antenna Skipping is the refraction and reflection of sky
waves
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Ionosphere
The layers that form the ionosphere varygreatly in altitude, density, and thickness withthe varying degrees of solar activity.
The upper portion of the F layer is mostaffected by sunspots or solar disturbances
There is a greater concentration of solar
radiation during peak sunspot activity. The greater radiation activity the more dense
the F layer and the higher the F layerbecomes and the greater the skip distance
http://www.ips.gov.au/papers/richard/hfreport/webrep.htmhttp://www.ips.gov.au/papers/richard/hfreport/webrep.htm8/2/2019 Advance Communication System Lectures Part 6
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FIGURE 12-11 Relationship of frequency to refraction by the ionosphere.
Gary M. Miller, Jeffrey S. BeasleyModern Electronic Communication, 7e
Copyright 2002 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
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FIGURE 12-12 Relationship of frequency to critical angle.
Gary M. Miller, Jeffrey S. BeasleyModern Electronic Communication, 7e
Copyright 2002 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
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Terms
Critical Frequency: The highest frequency that will be returned
to the earth when transmitted verticallyunder given ionospheric conditions
Critical Angle: The highest angle with respect to a vertical
line at which a radio wave of a specifiedfrequency can be propagated and still bereturned to the earth from the ionosphere
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Maximum usable frequency (MUF)
The highest frequency that is returned to
the earth from the ionosphere betweentwo specific points on earth
Optimum Working frequency:
The frequency that provides for the mostconsistent communication path via skywaves
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Quiet Zone or Skip Zone:
The space between the point where the
ground wave is completely dissipated andthe point where the first sky wave isreceived
Fading:Variations in signal strength that may occur
at the receiver over a period of time.
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Space Wave
Two types
Direct
Ground reflected
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FIGURE 12-6 Direct and ground reflected space waves.
Gary M. Miller, Jeffrey S. BeasleyModern Electronic Communication, 7e
Copyright 2002 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
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Direct
Limited to line-of sight transmissiondistances
Antenna height and curvature of earthare limiting factors
Radio horizon is about 80% greater
than line of sight because of diffractioneffects
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FIGURE 12-7 Radio horizon for direct space waves.
Gary M. Miller, Jeffrey S. BeasleyModern Electronic Communication, 7e
Copyright 2002 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458
All rights reserved.
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Reflected
Part of the signal from the transmitter isbounced off the ground and reflected
back to the receiving antenna Can cause problems if the phase
between the direct wave and thereflected wave are not in phase
Detuning the antenna so that thereflected wave is too weak to receive
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Tropospheric scattering
Tropospheric Scattering
Signals are aimed at the troposphere
rather than the ionosphere 350 Mhz to 10GHz for paths up to 400 mi
Received signal = 10-6 th of the
transmitted power Fading a problem
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T3-47
Line-Of-Sight Propagation
Radio signals travel in a straight linefrom a transmitting antenna to the
receiving antenna. Provides VHF/UHF communications
within a 100 miles or so.
Signals can be reflected by buildings,hills, airplanes, etc.
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Radio Path Horizon
The distance D to the radio horizon is greater from a higher
antenna. The maximum distance over which two stations may
communicate by space wave is equal to the sum of their
distances to the horizon.
VHF/UHF Si l Th h
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T3-49
VHF/UHF Signals ThroughIonosphere
Sporadic E
A type of sky-wave propagation that allows
long distance communication on the VHFbands (6 meters, 2 meters and 220 Mhz)through the E region of the atmosphere.
Occurs only sporadically during certain
times of the year.
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Radio SpectrumSymbol
Frequencyrange
Wavelength,
Comments
ELF < 300 Hz > 1000 km Earth-ionosphere waveguidepropagationULF 300 Hz 3 kHz 1000 100
km
VLF 3 kHz 30 kHz 100 10 km
LF 30 300 kHz 10 1 km Ground wave propagation
MF 300 kHz 3MHz
1 km 100m
HF 3 30 MHz 100 10 m Ionospheric sky-wave propagationVHF 30 300 MHz 10 1 m Space waves, scattering by objects
similarly sized to, or bigger than, afree-space wavelength, increasinglyaffected by tropospheric phenomena
UHF 300 MHz 3GHz
1 m 100mm
SHF 3 30 GHz 100 10 mm8 1