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Electromagnetic Waves and Radio Transmission

ELEC166 EM Waves

A wave is a disturbance which propagates through 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

ELEC166 EM Waves

Electromagnetic waves
Transverse waves, electric and magnetic fields varying 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

Wave properties
Wave travels in this direction with velocity v Water particles move up and down only

The number of peaks passing any point per second is the frequency (f)

Distance between successive peaks or troughs is the wavelength ()

Peak height of wave above the average is the amplitude (A)

ELEC166 EM Waves

Frequency, Velocity and Wavelength

Basic equation for all waves:

v = velocity (metres/sec) f = frequency (Hz) = wavelength (metres)

ELEC166 EM Waves

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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The Electromagnetic Spectrum

Common name Radio waves: (AM1 radio band) (FM2 radio band) (VHF3 TV band) (UHF4 TV band) Microwaves Infrared (IR) Visible light Ultraviolet (UV) X-rays Gamma rays Approx wavelength (in vacuum or air) 30km - 30cm 600m - 200m 3m 6.7m - 1.4m 0.57m - 0.37m 30cm - 1mm 1mm - 700nm ~700nm - 300nm 300nm - 100pm 1nm - 100fm < 100pm Approx frequency 10kHz - 1GHz 0.5 - 1.6 MHz 88 - 108 MHz 45 - 220 MHz 530 - 820 MHz 1GHz - 300GHz Notes very broad range, including four specific examples shown

includes "heat" radiation short wavelengths are "blue", long "red overlaps both UV and gamma rays

Terminology: 1: AM = amplitude modulation 2: FM = frequency modulation 3: VHF = very high frequency ( 30 - 300 MHz) 4: UHF = ultra high frequency ( 300 - 3000 MHz)
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Inverse Square Law

P I= 4 R2
(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-omnidirectional source Double distance intensity (-6 dB)
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Visualising the Inverse Square Law

Surface which radiation passes through increases in proportion to R2
Imaginary surfaces of spheres at distances R, 2R At distance R,radiation spread over area A


Source radiates in all directions

At distance 2R, same radiation spread over area 4A, so intensity 1/4 as great
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Example 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 shout loudly when using it The phone transmitter has an output power of 3 watts. Your radio amateur neighbour has a 1000 watt transmitter, with the antenna located about 50 metres from your bedroom. If we assume that both antennas are omnidirectional, and that the inverse square 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 the mobile phone: Intensity = 3 / (4 12) = 0.24 W/m2, which is about 8 times higher than the intensity produced by the highpower transmitter.
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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 absorb energy 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 and influential

ELEC166 EM Waves


Diffraction of Waves
Cant make exactly parallel beam of waves. It must diverge with increasing distance. Waves leak around edges, obstacles. Effect more obvious at longer wavelengths
Shorter wavelength waves create sharper shadow of object AM radio band waves (>100m) will diffract around hills, but UHF TV (~1m) will not.
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Diffraction of water waves

Waves roll in parallel to beach


Waves diffract around breakwater


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Diffraction of water waves


Obstacle many wavelengths wide

Obstacle less than 1 Wavelength wide

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Antenna converts electrical signal to an EM wave, or vice versa. Theoretically, always regarded as transmitting equations are the same. Many different types of antennas, but some principles are common to all.

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Antenna types

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Directionality of Antennas
Gain (dBi) =
intensity produced by antenna 10 log10 intensity produced by ideal omnidirectional antenna

Beamwidth is angle over which gain is within 3 dB of maximum. Beamwidth in radians is roughly equal to
wavelength largest dimension of antenna
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Antenna Gain
Measured relative to isotropic radiator (ideal omnidirectional antenna). Signals are boosted by an amount equal to the antenna gain in dB in that direction. Gain in some directions must be < 0 dBi (<1), since (for a transmitting antenna) the total power transmitted is just the sum over all directions.

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Radiation Pattern
Polar graph Beamwidth ~ 70 degrees.
Direction of maximum radiation Beamwidth (-3dB)
0 -10 -20 -30


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Circular Antennas
Approximate formulas:

wavelength Beamwidth 75 degrees antenna diameter

2 diameter Gain 10 log10 0.75 wavelength dBi

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Example 11.8: A ground station for a communications satellite operates an uplink (i.e. earth-to-satellite) transmitter at a frequency of about 14 GHz. It uses a circular dish antenna with a diameter of 25 metres. (a) Approximately what beamwidth would you expect the antenna to have? (b) What would you expect its gain to be? Answer: First we need to know the wavelength. At 14 GHz, the wavelength 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 = 0.063 degrees (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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Optical and Infrared Transmission

ELEC166 Optical and IR

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

ELEC166 Optical and IR

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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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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Focusing a light beam

Can produce a spot no smaller than about one wavelength in size To pack more information on optical discs, need smaller spot size and hence shorter wavelength lasers CD 780 nm (IR) DVD ~650 nm (red) Blu-ray 405 nm (blue)

ELEC166 Optical and IR

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 and off

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Optical fibre construction

Outer jacket for protection
(typically 0.125 mm diameter)

Glass core

Light travels through core

Fibre is drawn out from high-purity glass

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Light transmission through fibre

cladding core

Light travels in core Confined by total internal reflection at corecladding boundary, due to different refractive indices of core and cladding
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Dispersion in fibres
Spread in arrival time of signals (smearing) Modal dispersion Light can take many paths, each taking a slightly 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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Smearing of signals in time makes recovery difficult Limits bandwidth of fibre

Fibre Single pulse Pulse smeared in time

Fibre Sequence of pulses A problem ...!

ELEC166 Optical and IR


Fibre types
Multimode step index Large core, severe modal dispersion Rarely used for telecommunications Graded index fibre Refractive index varied across fibre so that light is continually refocussed Low modal dispersion Typical core diameter 50 m, 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 modal dispersion, need to use laser diode source Best performance, used in high data rate/long distance applications
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Fibre types (cont)

Multimode step index fibre

Multimode graded index fibre

Monomode step index fibre

ELEC166 Optical and IR


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 make cheaper fibres, with red LEDs as light sources Can be made reasonably fast

Experimental holey PMMA fibre (Macquarie Uni)

ELEC166 Optical and IR


Attenuation in fibres
Extremely low loss (few tenths of dB per km in best materials) Loss depends on wavelength, main loss due to OH 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)



0 800


1200 Wavelength (nm)



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Optical fibre cables

Many fibres run together, outer protective layers
Fibre in tube (1 of 8)

Central carrier

Shock resistant packaging

Reinforcement and protective layers

ELEC166 Optical and IR