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Lecture 2: Antennas and Propagation

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Effective Earth Radius Microwave Communication Line-of-Sight Range Fresnel Zone Ionospheric Communication Propagation Modelling Indoor models Dipole antenna Half ... – PowerPoint PPT presentation

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Title: Lecture 2: Antennas and Propagation


1
Lecture 2 Antennas and Propagation
  • Anders Västberg
  • vastberg_at_kth.se
  • 08-790 44 55

2
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
3
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

4
Radiation
Only accelerating charges produce radiation
Saunders, 1999
5
Electromagnetic Fields
Poyntings Vector
Power density
6
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
7
Free Space Propagation
8
Antenna Gain
  • The antenna gain is defined by its relative power
    density

9
Propagation between two antennas (not to scale)
No Ground Wave for Frequencies gt 2 MHz No
Ionospheric Wave for Frequencies gt 30 Mhz
10
Diffraction
Saunders, 1999
11
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
12
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
13
Effective Earth Radius
Slimane
14
Microwave Communication
Slimane
15
Line-of-Sight Range
Slimane
16
Fresnel Zone
Slimane
17
Ionospheric Communication
Davies, 1993
18
Propagation Modelling
Slimane
19
Indoor models
20
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
21
Corner Reflectors
  • Multiple images results in increased gain
  • ExampleG12 dBi

Images
l/2
Driven Element
22
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
23
Loop-antenna
  • Linear Polarisation
  • Gain 1,76 dBi

http//www.ycars.org/EFRA/Module20C/AntLoop.htm
24
Parabolic antenna
  • Effective area
  • Ae hp d2/4
  • h0.56

Stallings, 2005
25
Helical antenna
  • Normal mode
  • Axial mode

http//hastingswireless.homeip.net/index.php?page
antennastypehelical
26
Multipath propagation
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