Lecture 5: Antennas and Wave Propagation

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Lecture 5 Antennas and Wave Propagation

• Anders Västberg
• vastberg_at_kth.se
• 08-790 44 55

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Digital Communication System
Source of Information
Source Encoder
Modulator
RF-Stage
Channel Encoder
Digital Modulator
Channel
RF-Stage
Information Sink
Source Decoder
Demodulator
Channel Decoder
Digital Demodulator
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Maxwell's Equations

• Electrical field lines may either start and end
  on charges, or are continuous
• Magnetic field lines are continuous
• An electric field is produced by a time-varying
  magnetic field
• A magnetic field is produced by a time-varying
  electric field or by a current

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Radiation
Only accelerating charges produce radiation
Saunders, 1999
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Electromagnetic Fields
Poyntings Vector
Power density
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Impedance of Free Space

• Both fields carry the same amount of energy
• Free space impedance is given by
• The power density can be expressed as

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Isotropic Antenna

• An isotropic radiator is an hypothetical antenna
  that generates a uniform field, i.e., energy
  flows with equal strength in all directions.
• Let Pt be the total power emitted by this
  antenna.
• This total power will be uniformly distributed
  over the surface of a sphere enclosing the
  antenna.
• The power density on a sphere of radius r is
  given by

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Effective Aperture of an Antenna

• Ae is defined as the area of a perfect lossless
  antenna
• The received power can be written as follows
• Ae is dependent on the type of receiving antenna

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Antenna Gain

• The antenna gain is defined by its relative power
  density

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Wave propagation

• The field at the receiver can be decomposed into
  three components
• Direct wave, Line-of-Sight Path
• Ground reflected wave
• Ground Wave (less than 2 MHz, less than 10 MHz
  over water)

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Plane Earth Model
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Plane Earth Model
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Diffraction
Saunders, 1999
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Diffraction

• For radio wave propagation over rough terrain,
  the propagation is dependent on the size of the
  object encountered.
• Waves with wavelengths much shorter than the size
  of the object will be reflected
• Waves with wavelengths much larger than the size
  of the obstacle will pass virtually unaffected.
• Waves with intermediate wavelengths curve around
  the edges of the obstacles in their propagation
  (diffraction).
• Diffraction allows radio signals to propagate
  around the curved surface and propagate behind
  obstacles.

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Propagation in the Atmosphere

• The atmosphere around the earth contains a lot of
  gazes (1044 molecules)
• It is most dense at the earth surface (90 of
  molecules below a height of 20 km).
• It gets thinner as we reach higher and higher
  attitudes.
• The refractive index of the air in the atmosphere
  changes with the Height
• This affects the propagation of radio waves.
• The straight line propagation assumption may not
  be valid especially for long distances.

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Effective Earth Radius
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Microwave Communication
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Line-of-Sight Range
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Fresnel Zone
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Ionospheric Communication
Davies, 1993
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Propagation Modelling
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Propagation Modelling
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