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7/28/2019 Electromagnetic Waves and Radio Transmission.pdf
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ELEC166 EM Waves 1
Electromagnetic Waves andRadio Transmission
ELEC166 EM Waves 2
Waves
A wave is a disturbance which propagatesthrough a medium
Carries energy
Longitudinal waves Medium wobbles in direction of wave motion
eg compression waves in spring, sound waves
Transverse waves Medium wobbles at right angles to direction of wave motion
Eg water waves, ripples in a stretched string
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ELEC166 EM Waves 3
Electromagnetic waves
Transverse waves, electric and magnetic fieldsvarying
Will travel through a vacuum
Travel at the velocity of light (c)
300,000 km/sec in a vacuum (or air)
Common examples are light, radio waves
Basic principles of wave behaviour apply to all
types of waves
ELEC166 EM Waves 4
Wave properties
Wave travels in thisdirection with veloc i ty v
Water particles moveup and down only
Distance betweensuccessive peaks ortroughs is the ( )wavelength
Peak height of waveabove the average is
the (A)ampl i tude The number of peakspassing any point persecond is the f r e quency(f)
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ELEC166 EM Waves 5
Frequency, Velocity and Wavelength
Basic equation for all waves:
v = f
v = velocity (metres/sec)
f = frequency (Hz)
= wavelength (metres)
ELEC166 EM Waves 6
Example
Example 11.1: What is the wavelength (in air) of
(a) the radio waves broadcast by ABC-FM at 92.9 MHz, and
(b) the microwaves in a microwave oven (f = 2.45 GHz)?
Answer: Rearranging v = f, we have
= v / f, where v = c = 3.00 108 m/s, so that
(a) = 3.00 108 / (92.9 106) = 3.23 metres
(b) = 3.00 108 / (2.45 109) = 0.122 metres
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ELEC166 EM Waves 7
The Electromagnetic Spectrum
Terminology:
1: AM =amplitude modulation2: FM =frequency modulation
3: VHF =very high frequency( 30 - 300 MHz)
4: UHF =ultra high frequency( 300 - 3000 MHz)
Common name Approx wavelength(in vacuum or air)
App rox freq uenc y Notes
Radio waves: 30km - 30cm 10kHz - 1GHz very broad range,
(AM1 radio band) 600m - 200m 0.5 - 1.6 MHz including four
(FM2 radio band) 3m 88 - 108 MHz specific examples
(VHF3 TV band) 6.7m - 1.4m 45 - 220 MHz shown
(UHF4 TV band) 0.57m - 0.37m 530 - 820 MHz
Microwaves 30cm - 1mm 1GHz - 300GHz
Infrared (IR) 1mm - 700nm includes "heat" radiation
Visible light ~700nm - 300nm short wavelengths are "blue", long "red
Ultraviolet (UV) 300nm - 100pm
X-rays 1nm - 100fm overlaps both UV and gamma rays
Gamma rays < 100pm
ELEC166 EM Waves 8
Inverse Square Law
(omnidirectional source)
I = intensity (watts/m2) [intensity = power per area]P = source power (watts)
R = distance (m)
Applies only in free space
1/R2 part still works for non-omnidirectionalsource
Double distance intensity (-6 dB)
2R4
PI
=
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ELEC166 EM Waves 9
Visualising the Inverse Square Law
Surface which radiation passes through increases inproportion to R2
Imaginary surfaces ofspheres at distances R, 2R
Source radiatesin all directionsA4A
At distance R,radiationspread over area A
At distance 2R, same radiation spreadover area 4A, so intensity 1/4 as great
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ExampleExample 11.5: You have a mobile phone, but you don't like operating
it next to your head, so you hold it about a metre away and shoutloudly when using it The phone transmitter has an output power of 3watts. Your radio amateur neighbourhas a 1000 watt transmitter,with the antenna located about 50 metres from your bedroom. If weassume that both antennas are omnidirectional, and that the inversesquare law applies, which of the two radiation sources will produce
the greatest intensity at your head?
Answer: For the amateur radio transmitter:
Intensity = 1000 / (4 502) = 0.032 W/m2, while for themobile phone:
Intensity = 3 / (4 12) = 0.24 W/m2, which is about 8times higher than the intensity produced by the high-power transmitter.
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ELEC166 EM Waves 11
Absorption of EM Waves Inverse square law assumes no energy lost
as wave travels
However, the medium may absorb energy,converting it to heat
A slightly conductive medium will absorbenergy due to the small currents which flow
Microwave oven works on a similar principle, due
to presence of water in food Attenuation in dB proportional to distance
(just like a cable)
ELEC166 EM Waves 12
Absorption in the human body
Mobile phones probably constitute highest risk
Antenna cannot be shielded
Current Australian standard is 1 mW/cm2
(general public, averaged, over 2 GHz)
Non-heating effects may be important
Effects may take long time to emerge
Effects different at different wavelengths
Mobile phone industry is highly profitable andinfluential
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Diffraction of Waves
Cant make exactly parallel beamof waves. Itmust diverge with increasing distance.
Waves leakaround edges, obstacles.
Effect more obvious at longer wavelengths
Shorter wavelength waves create sharper shadowof object
AM radio band waves (>100m) will diffract aroundhills, but UHF TV (~1m) will not.
ELEC166 EM Waves 14
Diffraction of water waves
Waves roll in parallel to beach
Breakwater
Beach
Waves diffractaround breakwater
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Diffraction of water waves
Edge Obstacle many
wavelengths wide
Obstacle less than 1
Wavelength wide
ELEC166 EM Waves 16
Antennas
Antenna converts electrical signal to an EMwave, or vice versa.
Theoretically, always regarded as transmitting equations are the same.
Many different types of antennas, but someprinciples are common to all.
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ELEC166 EM Waves 17
Antenna types
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Directionality of Antennas
Gain (dBi) =
Beamwidth is angle over which gain is within 3dB of maximum.
Beamwidth in radians is roughly equal to
antennaionalomnidirectidealbyproducedintensity
antennabyproducedintensitylog10 10
antennaofdimensionlargest
wavelength
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Antenna Gain
Measured relative to isotropic radiator(idealomnidirectional antenna).
Signals are boosted by an amount equal to theantenna gain in dB in that direction.
Gain in some directions must be < 0 dBi (
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ELEC166 EM Waves 21
Circular Antennas
Approximate formulas:
degreesdiameterantenna
wavelength75Beamwidth
dBiwavelength
diameter0.75log10Gain
2
10
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ExampleExample 11.8: A ground station for a communications satellite
operates an uplink (i.e. earth-to-satellite) transmitter at afrequency of about 14 GHz. It uses a circular dish antenna with adiameter of 25 metres.
(a) Approximately what beamwidth would you expect theantenna to have?
(b) What would you expect its gain to be?
Answer: First we need to know the wavelength. At 14 GHz, thewavelength in air or vacuum will be = c / f, where c = 3.00 108
m/s and f = 14 109 Hz. This gives = 0.021 m.
(a) The beamwidth is thus approximately 75 0.021 / 25 = 0.063degrees (or about 4 minutes of arc).
(b) The gain in dB will be about 10 log10 (0.75 ( 25 / 0.021)2 )
= 70 dBi.
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ELEC166 Optical and IR 1
Optical and InfraredTransmission
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Optical and IR
Wavelength range about 300 nm to 1 mm
Range 650 nm to 1550 nm used for communication
Treated as light
Propagation methods Beams in free space (eg IR remote controls, IR links
for PDAs etc.)
Guided beams (optical fibres)
Semiconductor diodes used as light sources
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ELEC166 Optical and IR 3
Light emitting diodes (LEDs) Light produced at semiconductor junction
Used as indicator lamps
Colour depends on material
IR GaAs, red GaP, blue GaN
Radiation in fairly broad beam (tens of degrees)
Wide range of wavelengths (few % bandwidth)
Communications applications:
Remote controls ( = 950 nm)
IR links (eg IrDA for computers)
Lower speed optical fibres
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IR remote control
IR light
emitting diode
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ELEC166 Optical and IR 5
Laser diodes Solid-state lasers
Like LEDs, but more tricky
Narrower beam
Very small wavelength range
Small, can be focussed efficiently
Applications:
CD, DVD players
Surveying equipment, rangefinding
Optical radar speed guns
High-speed optical fibres
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IR laser diode in CD player
IR laser diode
With lens
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ELEC166 Optical and IR 7
Focusing a light beam
Can produce a spot no smaller than about onewavelength in size
To pack more information on optical discs, needsmaller spot size and hence shorter wavelengthlasers
CD 780 nm (IR)
DVD ~650 nm (red)
Blu-ray 405 nm (blue)
ELEC166 Optical and IR 8
Optical Fibres Replacing copper cables in many telecoms
networks
Large bandwidth (gives up to hundreds of Gbps)
Immunity to noise
High security
Electrical safety
Basically a light pipe
Information transmitted by turning light on andoff
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ELEC166 Optical and IR 9
Optical fibre construction
Fibre is drawn out from high-purity glass
Outer jacketfor protection
Glass core
Light travelsthrough core
(typically 0.125 mm diameter)
ELEC166 Optical and IR 10
Light transmission through fibre
Light travels in core
Confined by total internal reflection at core-cladding boundary, due to different refractiveindices of core and cladding
cladding
core
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ELEC166 Optical and IR 11
Dispersion in fibres Spread in arrival time of signals (smearing)
Modal dispersion
Light can take many paths, each taking aslightly different time
Chromatic dispersion
Material dispersion
Different wavelengths have different velocities
Common example in rainbows Important in high-quality lens design
Can make reduced dispersion fibres
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Dispersion
Smearing of signals in time makes recovery difficult
Limits bandwidth of fibre
Single pulse
Sequence of pulses
Fibre
Fibre
Pulse smeared in time
A problem ...!
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ELEC166 Optical and IR 13
Fibre types Multimode step index
Large core, severe modal dispersion
Rarely used for telecommunications
Graded index fibre
Refractive index varied across fibre so thatlight is continually refocussed
Low modal dispersion Typical core diameter 50m, use LEDs
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Fibre types (cont) Single mode (monomode)
Core has constant refractive index but very
small diameter (~10m)
Only one mode can propagate, so no modal
dispersion
(But more difficult to couple to light source)
To take advantage of absence of modaldispersion, need to use laser diode source
Best performance, used in high data rate/longdistance applications
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ELEC166 Optical and IR 15
Fibre types (cont)
Multimode step index fibre
Multimode graded index fibre
Monomode step index fibre
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Bandwidth To measure, consider light variations as analog
signal
Limited by dispersion (modal and chromatic)
Degree of smearing increases with fibre length
Hence for a particular fibre type
bandwidth distance = k (constant)
Typically
k = 200 to 1000 MHz-km for multimode fibre
k = 100 GHz-km for monomode fibre
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ELEC166 Optical and IR 17
PMMA fibres
Possible to use PMMA (perspex) to makecheaper fibres, with red LEDs as light sources
Can be made reasonably fast
Experimental holeyPMMA fibre(Macquarie Uni)
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Attenuation in fibres
Extremely low loss (few tenths of dB per km inbest materials)
Loss depends on wavelength, main loss due toOH ions in glass
3 commonly used windows
around 850nm, 1300nm and 1550nm
Better materials being developed
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ELEC166 Optical and IR 19
Attenuation in fibres
Attenuation(dB/km)
Wavelength (nm)800 1000 1200 1400 16000
1.0
2.0
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Optical fibre cables Many fibres run together, outer protective layers
Central carrier
Fibre in tube(1 of 8)
Shock resistantpackaging
Reinforcement andprotective layers
Recommended