Advance Communication System Lectures Part 6

8/2/2019 Advance Communication System Lectures Part 6

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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.htm


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FIGURE 12-11 Relationship of frequency to refraction by the ionosphere.





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FIGURE 12-12 Relationship of frequency to critical angle.





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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.





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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.





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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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T3-48

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

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Advance Communication System Lectures Part 6