lecture 2: antennas and propagation anders västberg [email protected] 08-790 44 55
TRANSCRIPT
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Lecture 2: Antennas and Propagation
Anders Västberg
08-790 44 55
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Digital Communication System
Source of Information
SourceEncoder
Modulator RF-Stage
Channel
RF-StageInformation
SinkSource
DecoderDemodulator
ChannelEncoder
DigitalModulator
ChannelDecoder
DigitalDemodulator
[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
)cos(}{),( tetrE tj EE
(V/m),2
1ErmsE
HEP
H2
1rmsH
)(W/m,2
1
2
1 2HEP S
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
H/m104
F/m10854185.87
0
120
22
0
HE
3770
00
Z
20
0
2
rmsrms HZ
Z
ES
[Slimane]
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Free Space Propagation
Ptr
Ae
2
2
4
4
r
APASP
r
PS
eterr
tr
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Antenna Gain
2
2
2
44
c
AfAG ee
• The antenna gain is defined by its relative power density
),(max SG
24
),(),,(
r
PS
SSrS
tr
rr
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Propagation between two antennas (not to scale)
No Ground Wave for Frequencies > ~2 MHzNo Ionospheric Wave for Frequencies > ~30 Mhz
Direct Wave
Ground ReflectedWave
Ground Wave
Sky Wave
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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
L=
I I
• 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
L=/4
I
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Corner Reflectors
• Multiple images results in increased gain
• Example:G=12 dBi
Driven Element
Images
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Yagi-antenna
http://www.urel.feec.vutbr.cz/~raida/multimedia_en/chapter-4/4_3A.html
3-30 element and a gain of 8-20 dBi
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Loop-antenna
http://www.ycars.org/EFRA/Module%20C/AntLoop.htm
• Linear Polarisation
• Gain 1,76 dBi
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Parabolic antenna
• Effective area
Ae = d2/4
[Stallings, 2005]
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Helical antenna
• Normal mode
• Axial mode
http://hastingswireless.homeip.net/index.php?page=antennas&type=helical
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Multipath propagation