Lecture 2: Antennas and Propagation

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Lecture 2 Antennas and 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
Slimane
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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

Slimane
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Free Space Propagation
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Antenna Gain

• The antenna gain is defined by its relative power
  density

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Propagation between two antennas (not to scale)
No Ground Wave for Frequencies gt 2 MHz No
Ionospheric Wave for Frequencies gt 30 Mhz
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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.

Slimane
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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.

Slimane
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Effective Earth Radius
Slimane
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Microwave Communication
Slimane
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Line-of-Sight Range
Slimane
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Fresnel Zone
Slimane
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Ionospheric Communication
Davies, 1993
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Propagation Modelling
Slimane
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Indoor models
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Dipole antenna

• Half-wave dipole
• Gain 1,64 2.15 dBi
• Linear Polarisation
• Quarter-wave dipole
• Conducting plane below a single quarter wave
  antenna. Acts like a half-wave dipole

Ll/4
I
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Corner Reflectors

• Multiple images results in increased gain
• ExampleG12 dBi

Images
l/2
Driven Element
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Yagi-antenna
3-30 element and a gain of 8-20 dBi
http//www.urel.feec.vutbr.cz/raida/multimedia_en
/chapter-4/4_3A.html
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Loop-antenna

• Linear Polarisation
• Gain 1,76 dBi

http//www.ycars.org/EFRA/Module20C/AntLoop.htm
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Parabolic antenna

• Effective area
• Ae hp d2/4
• h0.56

Stallings, 2005
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Helical antenna

• Normal mode
• Axial mode

http//hastingswireless.homeip.net/index.php?page
antennastypehelical
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Multipath propagation