radiation and propagation of waves chapter 10 standard text book lecture # 4

Radiation and Radiation and Propagation of Propagation of WavesWaves

Chapter 10Chapter 10

Standard Text BookStandard Text Book

Lecture # 4Lecture # 4

ObjectivesObjectives

Understand wave, electromagnetic waves Understand wave, electromagnetic waves Radiation and associated phenomenonRadiation and associated phenomenon ExplainExplain

ReflectionReflection RefractionRefraction DiffractionDiffraction PolarizationPolarization

Describe the Propagation of wavesDescribe the Propagation of waves Ground WavesGround Waves Sky wavesSky waves Space WavesSpace Waves

WaveWave

Wave is a mode of transfer of Wave is a mode of transfer of energyenergy

Transverse waves Transverse waves Longitudinal wavesLongitudinal waves

Transverse WavesTransverse Waves

Transverse waves are those whose Transverse waves are those whose direction of propagation is direction of propagation is perpendicular to both the electrical field perpendicular to both the electrical field and the magnetic field The electrical and the magnetic field The electrical field and the magnetic fields lie in field and the magnetic fields lie in planes that are perpendicular to each planes that are perpendicular to each other. (x and y planes)other. (x and y planes)

Thus the direction of propagation will be Thus the direction of propagation will be in the z plane or third dimensionin the z plane or third dimension

Electromagnetic WavesElectromagnetic Waves

Consist of Consist of Magnetic waveMagnetic wave Electrical waveElectrical wave

Most of the energy is returned to the circuit.Most of the energy is returned to the circuit. If it isn’t, then some it must be “set free” or If it isn’t, then some it must be “set free” or

radiated. Radiated energy is not desirable.radiated. Radiated energy is not desirable. But if such power is “escaped on purpose” But if such power is “escaped on purpose”

then it is said to be radiatedthen it is said to be radiated

Wave Propagation Wave Propagation ExampleExample

electricfield

magneticfield

propagation direction

Electrical to Magnetic Electrical to Magnetic ConversionConversion

The antennas are the transducersThe antennas are the transducers The transmitting antenna changes the The transmitting antenna changes the

electrical energy into electromagnetic or electrical energy into electromagnetic or waveswaves

The receiving antenna changes the The receiving antenna changes the electromagnetic energy back into electrical electromagnetic energy back into electrical energyenergy

These electromagnetic waves These electromagnetic waves propagate at rates ranging from 150kHz propagate at rates ranging from 150kHz to 300GHzto 300GHz

Radio-frequency Radio-frequency InterferenceInterference

If the radiated energy comes from another If the radiated energy comes from another radio transmitter, then it is considered radio transmitter, then it is considered radio-frequency interferenceradio-frequency interference (RFI) (RFI)

The transmitting antenna should be The transmitting antenna should be specifically designed to prevent the energy specifically designed to prevent the energy from being returned to the circuit.from being returned to the circuit.

It is desirable that the antenna “free” the It is desirable that the antenna “free” the energy in order that it might radiate into energy in order that it might radiate into spacespace

Electromagnetic Electromagnetic InterferenceInterference

If the energy comes from else If the energy comes from else where, then it is where, then it is electromagnetic electromagnetic interferenceinterference (EMI) (EMI)

A few Concepts at a A few Concepts at a glanceglance

Free SpaceFree Space Which does not interfere with normal radiation Which does not interfere with normal radiation

and propagationand propagation Point SourcePoint Source

A simple point acting like a source radiating in all A simple point acting like a source radiating in all directionsdirections

Power densityPower density Power per unit areaPower per unit area

Isotropic sourceIsotropic source one which radiates uniformly in all directionsone which radiates uniformly in all directions

Polarization of the Electrical Polarization of the Electrical FieldField

The polarization of the electrical The polarization of the electrical field is determined by the direction field is determined by the direction of oscillationsof oscillations If the oscillations are in the vertical If the oscillations are in the vertical

direction then the polarization is said direction then the polarization is said to be verticalto be vertical

If the oscillations are in the horizontal If the oscillations are in the horizontal direction then the polarization is said direction then the polarization is said to be horizontalto be horizontal

Polarization of antennaPolarization of antenna

Thus a “vertical” antenna will result Thus a “vertical” antenna will result in a vertically polarized wave.in a vertically polarized wave.

A vertical antenna is one that A vertical antenna is one that consists of a vertical tower, wire, or consists of a vertical tower, wire, or rod, usually a quarter wavelength rod, usually a quarter wavelength in length that is fed at the ground in length that is fed at the ground and uses the ground as a reflecting and uses the ground as a reflecting surface.surface.

WavefrontsWavefronts

A wavefront is a plane joining all A wavefront is a plane joining all points of equal phase in a wavepoints of equal phase in a wave

Take a point in space. Imagine Take a point in space. Imagine waves radiating outward in all waves radiating outward in all directions from this point. The directions from this point. The result would resemble a sphere. result would resemble a sphere. The point of radiation is called the The point of radiation is called the isotropic point sourceisotropic point source

Isotropic PowerIsotropic Power

Since the power at any point away from Since the power at any point away from the isotropic point is inversely the isotropic point is inversely proportional to the square of the distance proportional to the square of the distance from the point, then the power decreases from the point, then the power decreases rapidly the further away from the point rapidly the further away from the point you need.you need.

Although the wavefront is curved in Although the wavefront is curved in shape, from a distance small sections shape, from a distance small sections appear planar and can be thought of as appear planar and can be thought of as plane wavefrontsplane wavefronts

ReflectionReflection

Reflection is the abrupt reversal in Reflection is the abrupt reversal in directiondirection

Caused by any conductive medium Caused by any conductive medium such assuch as Metal surfaces orMetal surfaces or Earth’s surfaceEarth’s surface

There will normally be a shift in phaseThere will normally be a shift in phase Coefficient of reflection is less than 1Coefficient of reflection is less than 1

Complete ReflectionComplete Reflection

Complete reflection will occur only Complete reflection will occur only in perfect conductors and when in perfect conductors and when the electric field is perpendicular the electric field is perpendicular to the reflecting element or to the reflecting element or mediummedium

Coefficient of Reflection will be 1Coefficient of Reflection will be 1 Coefficient of Reflection is the ratio Coefficient of Reflection is the ratio

of the reflected wave intensity to of the reflected wave intensity to the incident wave intensitythe incident wave intensity

RefractionRefraction

Occurs when the waves pass from Occurs when the waves pass from one medium to another whose one medium to another whose densities are differentdensities are different

Coefficient of reflection is less than Coefficient of reflection is less than 11

The angle of incidence and the The angle of incidence and the angle of refraction is related by angle of refraction is related by Snell’s LawSnell’s Law

RefractionRefraction

RefractionRefraction (or bending) of signals is due to temperature, pressure, and (or bending) of signals is due to temperature, pressure, and water vapor content in the atmosphere.water vapor content in the atmosphere.

Amount of refractivity depends on the height above ground.Amount of refractivity depends on the height above ground. Refractivity is usually largest at low elevations.Refractivity is usually largest at low elevations. The refractivity gradient (k-factor) usually causes microwave signals to The refractivity gradient (k-factor) usually causes microwave signals to

curve slightly downward toward the earth, making the radio horizon father curve slightly downward toward the earth, making the radio horizon father away than the visual horizon.away than the visual horizon.

This can increase the microwave path by about 15%,This can increase the microwave path by about 15%,

Normal Refraction

Refraction (straight line)

Sub-Refraction

DiffractionDiffraction

Waves traveling in straight lines bend Waves traveling in straight lines bend around obstaclesaround obstacles

Based on Huygen’s principle (1690)Based on Huygen’s principle (1690) Each point on a wavefront can be thought of Each point on a wavefront can be thought of

as an isotropic point or a source of as an isotropic point or a source of secondary spherical energysecondary spherical energy

Concepts explains why radio waves can Concepts explains why radio waves can be heard behind tall mountains or be heard behind tall mountains or buildings that are normally considered buildings that are normally considered to block line of sight transmissionsto block line of sight transmissions

Attenuation and ObstructionsAttenuation and Obstructions

Longer the wavelengthLonger the wavelength (lower frequency) of the wireless (lower frequency) of the wireless signal, the signal, the lessless the signal is attenuated. 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.

PropagationPropagation……....

Ground and Space Ground and Space WavesWaves

Ground-Wave PropagationGround-Wave Propagation

The curved surface of the Earth horizon can diffract long-wavelength(low frequency) radio waves. The waves can follow the curvature of theEarth for as much as several hundred miles.

Ground-Wave PropagationGround-Wave Propagation Results from a radio wave diffraction along the Results from a radio wave diffraction along the

Earth’s surface.Earth’s surface. Primarily affects longer wavelength radio waves that Primarily affects longer wavelength radio waves that

have vertical polarization (electric field is oriented have vertical polarization (electric field is oriented vertically).vertically).

Most noticeable on AM broadcast band and the 160 Most noticeable on AM broadcast band and the 160 meter and 80 meter amateur bands.meter and 80 meter amateur bands.

Communication distances often extend to 120 miles Communication distances often extend to 120 miles or more.or more.

Most useful during the day at 1.8 MHz and 3.5 MHz Most useful during the day at 1.8 MHz and 3.5 MHz when the D-Region absorption makes sky-wave when the D-Region absorption makes sky-wave propagation impossible.propagation impossible.

Attenuation related to frequencyAttenuation related to frequency Loses increase with increase in Loses increase with increase in

frequencyfrequency Not very effective at frequencies Not very effective at frequencies

above 2Mhzabove 2Mhz Very reliable communication linkVery reliable communication link Reception is not affected by daily or Reception is not affected by daily or

seasonal weather changes seasonal weather changes

Used to communicate with Used to communicate with submarinessubmarines

ELF (30 to 300 Hz) propagation is ELF (30 to 300 Hz) propagation is utilizedutilized

Sky Wave Sky Wave PropagationPropagation

Atmospheric RegionsAtmospheric Regions

Region Height Notes

Troposphere 7 miles Region where all weather occurs

Stratosphere 6 to 30 miles

Region where atmospheric gases “spread out” horizontally. The high speed jet stream travels in the stratosphere.

Ionosphere 30 to 400 miles

Region where solar radiation from the sun creates ions. Major influence on HF radio wave propagation.

Atmospheric RegionsAtmospheric Regions

What is the ionosphere?What is the ionosphere?

The The ionosphereionosphere is the uppermost is the uppermost part of the atmosphere, part of the atmosphere, distinguished because it is distinguished because it is ionizedionized by by solar radiation.solar radiation.

At heights above 80 km (50 miles), At heights above 80 km (50 miles), the atmosphere is so thin that free the atmosphere is so thin that free electrons can exist for short periods electrons can exist for short periods of time before they are captured by of time before they are captured by nearby ions.nearby ions.

This part of the atmosphere is This part of the atmosphere is ionized and contains a ionized and contains a plasmaplasma..

In a plasma, negative free electrons In a plasma, negative free electrons and positive ions are attached by the and positive ions are attached by the electromagnetic force, but they are electromagnetic force, but they are too energetic to stay fixed together too energetic to stay fixed together in neutral molecules.in neutral molecules.

Sky-wave PropagationSky-wave Propagation

Ionization levels in the Earth’s Ionization levels in the Earth’s ionosphere can refract (bend) radio ionosphere can refract (bend) radio waves to return to the surface.waves to return to the surface. Ions in the Earth’s upper atmosphere are Ions in the Earth’s upper atmosphere are

formed when ultraviolet (UV) radiation and formed when ultraviolet (UV) radiation and other radiation from the sun knocks other radiation from the sun knocks electrons from gas atoms.electrons from gas atoms.

The ionization regions in the Earth’s The ionization regions in the Earth’s ionosphere is affected the sunspots on the ionosphere is affected the sunspots on the sun’s surfacesun’s surface

FIGURE 12-9FIGURE 12-9 Sky-wave propagation. Sky-wave propagation.

Gary M. Miller, Jeffrey S. Gary M. Miller, Jeffrey S. BeasleyBeasleyModern Electronic Communication, Modern Electronic Communication, 7e7e

Sky Wave PropagationSky Wave Propagation

Radio waves radiated from the transmitting Radio waves radiated from the transmitting antenna in a direction toward the antenna in a direction toward the ionosphereionosphere

Long distance transmissionsLong distance transmissions Sky wave strike the ionosphere, is refracted Sky wave strike the ionosphere, is refracted

back to ground, strike the ground, reflected back to ground, strike the ground, reflected back toward the ionosphere, etc until it back toward the ionosphere, etc until it reaches the receiving antennareaches the receiving antenna

Skipping is the refraction and reflection of Skipping is the refraction and reflection of sky wavessky waves

Ionosphere The layers that form the ionosphere vary The layers that form the ionosphere vary

greatly in altitude, density, and thickness greatly in altitude, density, and thickness with the varying degrees of solar activity.with the varying degrees of solar activity.

The upper portion of the F layer is most The upper portion of the F layer is most affected by sunspots or solar disturbancesaffected by sunspots or solar disturbances

There is a greater concentration of solar There is a greater concentration of solar radiation during peak sunspot activity.radiation during peak sunspot activity.

The greater radiation activity the more The greater radiation activity the more dense the F layer and the higher the F layer dense the F layer and the higher the F layer becomes and the greater the skip distancebecomes and the greater the skip distance

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

FIGURE 12-12FIGURE 12-12 Relationship of frequency to critical angle. Relationship of frequency to critical angle.

TermsTerms

Critical Frequency:Critical Frequency: The highest frequency that will be The highest frequency that will be

returned to the earth when transmitted returned to the earth when transmitted vertically under given ionospheric vertically under given ionospheric conditionsconditions

Critical Angle:Critical Angle: The highest angle with respect to a The highest angle with respect to a

vertical line at which a radio wave of a vertical line at which a radio wave of a specified frequency can be propagated specified frequency can be propagated and still be returned to the earth from the and still be returned to the earth from the ionosphereionosphere

Maximum usable frequency (MUF)Maximum usable frequency (MUF) The highest frequency that is returned The highest frequency that is returned

to the earth from the ionosphere to the earth from the ionosphere between two specific points on earthbetween two specific points on earth

Optimum Working frequency:Optimum Working frequency: The frequency that provides for the The frequency that provides for the

most consistent communication path most consistent communication path via sky wavesvia sky waves

Quiet Zone or Skip Zone:Quiet Zone or Skip Zone: The space between the point where The space between the point where

the ground wave is completely the ground wave is completely dissipated and the point where the first dissipated and the point where the first sky wave is receivedsky wave is received

Fading:Fading: Variations in signal strength that may Variations in signal strength that may

occur at the receiver over a period of occur at the receiver over a period of time.time.

Space WaveSpace Wave

Two typesTwo types Direct Direct Ground reflectedGround reflected

FIGURE 12-6FIGURE 12-6 Direct and ground reflected space waves. Direct and ground reflected space waves.

DirectDirect

Limited to “line-of sight” Limited to “line-of sight” transmission distancestransmission distances

Antenna height and curvature of Antenna height and curvature of earth are limiting factorsearth are limiting factors

Radio horizon is about 80% greater Radio horizon is about 80% greater than line of sight because of than line of sight because of diffraction effects diffraction effects

FIGURE 12-7FIGURE 12-7 Radio horizon for direct space waves. Radio horizon for direct space waves.

ReflectedReflected

Part of the signal from the Part of the signal from the transmitter is bounced off the transmitter is bounced off the ground and reflected back to the ground and reflected back to the receiving antennareceiving antenna

Can cause problems if the phase Can cause problems if the phase between the direct wave and the between the direct wave and the reflected wave are not in phasereflected wave are not in phase

Detuning the antenna so that the Detuning the antenna so that the reflected wave is too weak to receivereflected wave is too weak to receive

Tropospheric scatteringTropospheric scattering

Tropospheric ScatteringTropospheric Scattering Signals are aimed at the troposphere Signals are aimed at the troposphere

rather than the ionosphererather than the ionosphere 350 Mhz to 10GHz for paths up to 400 350 Mhz to 10GHz for paths up to 400

mimi Received signal = 10Received signal = 10-6-6 th of the th of the

transmitted powertransmitted power Fading a problemFading a problem

Line-Of-Sight PropagationLine-Of-Sight Propagation

Radio signals travel in a straight Radio signals travel in a straight line from a transmitting antenna to line from a transmitting antenna to the receiving antenna.the receiving antenna.

Provides VHF/UHF communications Provides VHF/UHF communications within a 100 miles or so.within a 100 miles or so.

Signals can be reflected by Signals can be reflected by buildings, hills, airplanes, etc. buildings, hills, airplanes, etc.

Radio Path HorizonRadio Path Horizon

The distance D to the radio horizon is greater from a higherantenna. The maximum distance over which two stations maycommunicate by space wave is equal to the sum of theirdistances to the horizon.

VHF/UHF Signals Through VHF/UHF Signals Through IonosphereIonosphere

Sporadic ESporadic E A type of sky-wave propagation that A type of sky-wave propagation that

allows long distance communication allows long distance communication on the VHF bands (6 meters, 2 meters on the VHF bands (6 meters, 2 meters and 220 Mhz) and 220 Mhz) through the E regionthrough the E region of the atmosphere.of the atmosphere.

Occurs only sporadically during Occurs only sporadically during certain times of the year.certain times of the year.

Radio SpectrumRadio SpectrumSymbol

Frequency range

Wavelength,

Comments

ELF < 300 Hz > 1000 km Earth-ionosphere waveguide propagationULF 300 Hz – 3

kHz1000 – 100 km

VLF 3 kHz – 30 kHz

100 – 10 km

LF 30 – 300 kHz 10 – 1 km Ground wave propagation

MF 300 kHz – 3 MHz

1 km – 100 m

HF 3 – 30 MHz 100 – 10 m Ionospheric sky-wave propagation

VHF 30 – 300 MHz 10 – 1 m Space waves, scattering by objects similarly sized to, or bigger than, a free-space wavelength, increasingly affected by tropospheric phenomena

UHF 300 MHz – 3 GHz

1 m – 100 mm

SHF 3 – 30 GHz 100 – 10 mm

EHF 30 – 300 GHz 10 – 1 mm

8 1; 3 10 msc f c

radiation and propagation of waves chapter 10 standard text book lecture # 4

electromagnetic energy

point of radiation

radiated energy

direction of propagation

vertical antenna

vertical direction

antenna free

propagation of waveschapter

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