EM Waves

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EM Waves
MCS.01
Maj JW Paul
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Education is what you get from reading the fine
print
Experience is what you get from not reading it...
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Review

• What are the components of the communication
  model?

Aside multiplexer
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Review - Transmission

• signal source microphone, musical instrument,
  data source
• source encoder produces an output that is
  compatible with the communication channel (e.g.,
  D/A, electrical/electromagnetic).
• Encryptor to assure a secure communication. Only
  the intended receiver can understand the message
  and only the authorized sender can transmit it
  (e.g., scrambling systems).
• The channel encoder increases the system
  performance. (e.g., In digital communication, the
  encoding permits error correction).
• The modem modifies the range of signal frequency
  to permit efficient transmission within the
  channel bandwidth and permit multiple use of the
  channel. Spread spectrum modulation is also used
  to provide some immunity to interference and
  fading effects.

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Review - Modulation

• It is often necessary to modify a low-frequency
  signal before injecting it into the channel.
• It is desirable to transmit more than one analog
  signal at a time.
• Efficient transmission and reception of radio
  waves frequencies is not practical due to the
  large antennas required.
• A quarter-wavelength antenna, at 3 kHz, would be
  25 km long.
• A quarter-wavelength antenna, at 2 GHz, would be
  3.75 cm long.

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Todays Class

• Electromagnetic Fields
• Electric Field
• Magnetic Field
• Polarized Fields
• Formula
• Antenna
• how they affect EM fields

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EM Fields
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Electric Magnetic Fields

• What is a FIELD?
• a quantity (scalar, vector) which is defined at
  each point in space
• static (does not depend on time)
• dynamic (time varying)

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Static EM Fields
Electric field around a static positive (red) and
negative charge (blue)
Magnetic field around a permanent magnet
There is no interaction between static electric
and static magnetic fields
So why do we talk about Electromagnetic Fields?
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Electromagnetic Field

• What happens if the electric or magnetic field is
  dynamic (ie changing with time)

Direction of motion
Force direction
A flowing current generates a magnetic field
A moving magnetic field induces a current in a
conducting loop. A voltage (electric field) is
induced in the loop.
Aside - Right Hand Rule
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Dynamic EM Fields

• A magnetic field which changes with time induces
  its own electric field
• An electric field which changes with time induces
  its own magnetic field

An Electromagnetic Field Has both a magnetic and
an electric field. One can not exist without the
other as long as they depend on time
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Characteristics of waves

• Frequency f Hertz (Hz), KHz,
• number of changes per second
• Period T seconds
• time it takes for a wave to repeat itself
• Wavelength ? meter, cm,
• distance travelled by the wave in one period
• Velocity v m/s
• velocity of the wave front

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Waves
v
1
? v T
f
f
T
?
for EM waves v c c 3x108 m/s in a vacuum
Aside - types of waves compression transverse circ
ular
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EM Waves

• Keep in mind EM waves are made up of an electric
  and magnetic field

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Polarization

• Defined by orientation of the electric field
• Linear Polarization
• direction of the electric component remains
  constant with the time at a fixed point in space

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Circular Polarization

• the vector of electric field has constant
  magnitude but its direction in the plane normal
  to the direction of propagation rotates smoothly
  through 3600 per wavelength of propagation. Can
  be left handed or right handed

Amplitude of electric field is not constant
Elliptical
Circular
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Properties of EM waves

• velocity speed of light (in vacuum)
• atmosphere is an in-homogeneous time-dependent
  medium
• waves are propagated, scattered, reflected,
  attenuated in atmosphere
• the power density of a wave propagating in open
  space decreases as 1/R2 even with no losses !!

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What about transmission lines

• Copper cable, fibre optics, wave guides
• Still lossy-media - implies attenuation
• Can get skin effects
• More on this later

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Electromagnetic Spectrum
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Terminonolgy

• Frequency Designation Abbreviation
• 30-300 Hz Extremely low frequency ELF
• 300-3000 Hz Voice frequency VF
• 3-30 kHz Very low frequency VLF
• 30-300 kHz Low frequency LF
• 300 kHz-3 MHz Medium frequency MF
• 3-30 MHz High frequency HF
• 30-300 MHz Very high frequency VHF
• 300 MHz-3 GHz Ultra high frequency UHF
• 3-30 GHz Super high frequency SHF
• 30-300 GHz Extra high frequency EHF

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Absorption
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Experience is something you get just after you
need it...
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EM Spectrum
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Antennas

• What
• Why
• How
• Types

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What

• An antenna is defined as an efficient radiator of
  EM energy (radio waves) into free space.
• The purpose of a transmitting antenna is to
  radiate as much power (watts or kilowatts) as
  possible, either in all directions
  (omnidirectional antenna) or in a specified
  direction (directional antenna).
• A receiving antenna is used to intercept
  electromagnetic waves and deliver RF signal
  voltage for further processing.

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Antenna Aperture

• Power density of plane wave W0
• Surface area of horn A
• Power captured by horn Pr
• Ideal situation Pr W0 x A
• Reality Pr W0 x A x ?a. ?a 0.5-0.7
• Effective aperture Aeff A x ?a
• The larger the effective area of an antenna the
  more energy it can capture

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Why

• Isotropic source
• An isotropic source is an antenna that radiates
  energy uniformly in all directions
• W total radiated power
• plus have attenuation
• Keep in mind - transmitting and receiving Ae

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Directivity and Gain

• Directivity
• D maximum radiation intensity
• isotropic baseline intensity
• Gain
• G maximum radiation intensity
• reference Ae intensity

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Beam Pattern
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Whip Antenna
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Horn Antenna

• The maximum radiation and response coincides with
  the axis of the horn. It is usually fed with a
  waveguide.
• Efficient radiators when wavelength comparable to
  aperture size.
• Used to feed reflector antennas (dish),
  measurement standards, short-range radar
  (police).

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Radiation Pattern of a Horn Antenna
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Reflector Antenna

• High Gain Antenna
• Dimensions much larger than ?
• Used in .

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Helical Antenna

• High gain with enough loops
• Circular, elliptical and linear polarization
• Used in space probes, telemetry, tracking

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Microstrip Antenna

• Low to moderate gain
• Narrow Bandwidth
• Linear or Circular Polarization
• Used on space craft, cell phones

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Loop Antenna

• Used mostly in receiving mode because of their
  inefficiency as radiators

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Antenna Arrays

• Problem - a single radiator
• May not be sufficient for some applications.
• Radiation characteristics fixed.
• Basic idea
• Adjust feeding (magnitude and phase) of array
  elements to achieve desired radiation
  characteristics.
• Applications
• Wireless Communications, Radar, Astronomy (high
  gain and steering)

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Control of Arrays

• Geometric configuration
• circular, linear
• Relative displacement between elements
• Amplitude of each element
• Phase of each element
• Pattern of each element

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Review

• Describe an EM wave
• How do frequency, period and wavelength relate
• What is the purpose of an antenna?

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Questions?