Electromagnetic Waves and Radio Transmission.pdf

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

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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)

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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

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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)

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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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ELEC166 EM Waves 13

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.

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Diffraction of water waves

Waves roll in parallel to beach

Breakwater

Beach

Waves diffractaround breakwater


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ELEC166 EM Waves 15

Diffraction of water waves

Edge Obstacle many

wavelengths wide

Obstacle less than 1

Wavelength wide

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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


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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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


IR remote control

IR light

emitting diode


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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


IR laser diode in CD player

IR laser diode

With lens


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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)


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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Optical fibre construction

Fibre is drawn out from high-purity glass

Outer jacketfor protection

Glass core

Light travelsthrough core

(typically 0.125 mm diameter)


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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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


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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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


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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Fibre types (cont)

Multimode step index fibre

Multimode graded index fibre

Monomode step index fibre


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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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)


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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Attenuation in fibres

Attenuation(dB/km)

Wavelength (nm)800 1000 1200 1400 16000

1.0

2.0


Optical fibre cables Many fibres run together, outer protective layers

Central carrier

Fibre in tube(1 of 8)

Shock resistantpackaging

Reinforcement andprotective layers

Electromagnetic Waves and Radio Transmission.pdf

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