Antenna Fundamentals - Wireless
Transcript of Antenna Fundamentals - Wireless
Antenna Fundamentals
Prof. Ryszard Struzak Na8onal Ins8tute of Telecommunica8ons, Poland
r.struzakATieee.org
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• Beware of misprints!!! These materials are preliminary notes intended for my lectures only and may contain misprints. Feedback is welcome. If you no8ce serious faults, or you have an improvement sugges8on, please describe it exactly and I will try to modify the materials.
• This work is licensed under the Crea8ve Commons ANribu8on License (hNp://crea8vecommons.org/ licenbses/by/1.0) and may be used freely for individual study, research, and educa8on in not-‐for-‐profit applica8ons. Any other use requires the wriNen author’s permission. If you cite these materials, please credit the author, 8tle, and place.
• These materials and any part of them may not be published, copied to or issued from another Web server without the author's wriNen permission.
• Copyright © 2012 Ryszard Struzak.
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Objec8ve
• to refresh basic concepts related to the antenna physics – needed to understand beNer the opera8on and design of microwave links and networks
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Antennas for laptop applica8ons
Source: D. Liu et al.: Developing integrated antenna subsystems for laptop computers; IBM J. RES. & DEV. VOL. 47 NO. 2/3 MARCH/MAY 2003 p. 355-‐367
Linksys
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Antenna func8ons
• Transforma8on of a guided EM wave into an unguided wave freely propaga8ng in space (or the opposite) – from 8me-‐func8on in one-‐dimensional space into 8me-‐func8on in 3-‐D space
– The specific form of the radiated wave is defined by the antenna structure and the environment
Space wave
Guided wave
7
• The junc8on – power reflec8ons, use matching devices
• Radiator – Must radiate efficiently – must be of a size comparable with the half-‐wavelength
• Resonaces – Unavoidable -‐ for broadband applica8ons resonances must be aNenuated
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Transmiong antenna equivalent circuit
TransmiNer Transm. line
Antenna G
ener
ator
RG
jXG
VG
jXA
Rr
Rl
The transmiNer with the transmission line is represented by an (Thevenin) equivalent generator
The antenna is represented by its input impedance
(which is frequency-‐dependent and is influenced by objects nearby) as seem from the generator
jXA represents energy stored in electric (Ee) and magne8c
(Em) near-‐field components; if |Ee| = |Em| then XA = 0 (antenna resonance) Can be approximated by the impedance of a transmission line
Rr represents energy radiated into space (far-‐field
components)
Rl represents energy lost, i.e. transformed into heat in the antenna structure
Radio wave
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Receiving antenna equivalent circuit A
nten
na
Rr
jXA
VA
jXL
RL Rl
Thevenin equivalent
The antenna with the transmission line is represented by an (Thevenin) equivalent generator The receiver is represented by its input impedance as seen from the antenna terminals (i.e. transformed by the transmission line) VA is the (induced by the incident wave) voltage at the antenna terminals determined when the antenna is open circuited Note: The antenna impedance is the same when the antenna is used to radiate and when it is used to receive energy
Radio wave Receiver Transm.line
Antenna
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Power transfer
• The maximum power is delivered to (or from) the antenna when the antenna impedance and the impedance of the equivalent generator (or load) are matched
0
0.5
1
0.1 1 10
RA / RG; (XA+XG = 0)
PA
/ P
Am
ax
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• When the impedances are matched – Half of the source power is delivered to the load and half is dissipated within the (equivalent) generator as heat
– In the case of receiving antenna, a part (Pl) of the power captured is lost as heat in the antenna elements, , the other part being reradiated (scaNered) back into space
• Even when the antenna losses tend to zero, s8ll only half of the power captured is delivered to the load (in the case of conjugate matching), the other half being scaNered back into space
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• When the antenna impedance is not matched to the transmiNer output impedance (or to the receiver input impedance) or to the transmission line between them, impedance-‐matching devices must be used for maximum power transfer
• Inexpensive impedance-‐matching devices are usually narrow-‐band
• Transmission lines oten have significant losses
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Radia8on efficiency
• The radia8on efficiency e indicates how efficiently the antenna uses the RF power
• It is the ra8o of the power radiated by the antenna and the total power delivered to the antenna terminals (in transmiong mode). In terms of equivalent circuit parameters:
rrlReRR=+
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EM Field of Current Element
!"
!"
HHHH
EEEE
r
r!!!!
!!!"
++=
++=
I: current (monochromatic) [A]; dz: antenna element (short) [m] x
y
z
θ
ϕ
OP
r
Er Eθ
Eϕ
I, dz
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Short dipole antenna: summary • Eθ & Hθ are maximal in the equatorial plane, zero along the antenna axis • Er is maximal along the antenna axis dz, zero in the equatorial plane • All show axial symmetry • All are propor8onal to the current moment Idz • Have 3 components that decrease with the distance-‐to-‐wavelength ra8o
as – (r/λ)-‐2 & (r/λ)-‐3: near-‐field, or induc8on field. The energy oscillates from
en8rely electric to en8rely magne8c and back, twice per cycle. Modeled as a resonant LC circuit or a transmission-‐line resonator;
– (r/λ)-‐1: far-‐field or radia8on field – These 3 component are all equal at (r/λ) = 1/(2π)
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Field components
0.001
0.01
0.1
1
10
100
1000
0.1 1 10
Relative distance, Br
Rela
tive
field
stre
ngth
FF
FF
Q
Q
C
C
FF: Radiation field
C, Q: Induction fields
β
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Field impedance
Field impedance Z = E/H depends on the antenna type and on distance
0.01
0.1
1
10
100
0.01 0.1 1 10 100
Distance / (lambda/ 2Pi)
Z / 3
77
Short dipole
Small loop
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Far-‐Field, Near-‐Field
• Near-‐field region: – Angular distribu8on of energy depends on
distance from the antenna; – Reac8ve field components dominate (L, C)
• Far-‐field region: – Angular distribu8on of energy is independent on
distance; – Radia8ng field component dominates (R) – The resultant EM field can locally be treated as
uniform (TEM)
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Poyn8ng vector
• The 8me-‐rate of EM energy flow per unit area in free space is the Poyn1ng vector (see hNp://www.amanogawa.com/archive/docs/EM8.pdf).
• It is the cross-‐product (vector product, right-‐hand screw direc8on) of the electric field vector (E) and the magne8c field vector (H): P = E x H.
• For the elementary dipole Eθ ⊥ Hθ and only EθxHθ carry energy into space with the speed of light.
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Power Flow
• In free space and at large distances, the radiated energy streams from the antenna in radial lines, i.e. the Poyn8ng vector has only the radial component in spherical coordinates.
• A source that radiates uniformly in all direc8ons is an isotropic source (radiator, antenna). For such a source the radial component of the Poyn8ng vector is independent of θ and ϕ.
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Linear Antennas • Summa8on of all vector
components E (or H) produced by each antenna element
• In the far-‐field region, the vector components are parallel to each other
• Phase difference due to – Excita8on phase difference – Path distance difference
• Method of moments -‐ NEC
...
...
321
321
+++=
+++=
HHHH
EEEE!!!!
!!!"
O
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Point Source
• For many purposes, it is sufficient to know the direc8on (angle) varia8on of the power radiated by antenna at large distances.
• For that purpose, any prac8cal antenna, regardless of its size and complexity, can be represented as a point-‐source.
• The actual field near the antenna is then disregarded.
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• The EM field at large distances from an antenna can be treated as originated at a point source -‐ fic88ous volume-‐less emiNer.
• The EM field in a homogenous unlimited medium at large distances from an antenna can be approximated by an uniform plane TEM wave
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Antenna radia8on paNern is 3-‐dimensional The 3-‐D plot assumes both angles θ and ϕ varying Difficult to produce and to interpret
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2-‐D paNern
Two 2-‐D paNerns
• Usually the antenna paNern is presented as a 2-‐D plot, with only one of the direc8on angles, θ or ϕ varies
• It is an intersec8on of the 3-‐D one with a given plane – usually it is a θ = const plane or
a ϕ= const plane that contains the paNern’s maximum
Source: NK Nikolova
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Isotropic antenna • Isotropic antenna or isotropic radiator is a hypothe8cal (not physically realizable) concept, used as a useful reference to describe real antennas.
• Isotropic antenna radiates equally in all direc8ons. – Its radia8on paNern is represented by a sphere whose center coincides with the loca8on of the isotropic radiator.
Source: NK Nikolova
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Anisotropic sources: gain • Every real antenna radiates more energy
in some direc8ons than in others (i.e. has direc8onal proper8es)
• Idealized example of direc8onal antenna: the radiated energy is concentrated in the yellow region (cone).
• Direc8ve antenna gain: the power flux density is increased by (roughly) the inverse ra8o of the yellow area and the total surface of the isotropic sphere – Gain in the field intensity may also be
considered -‐ it is equal to the square root of the power gain.
Hypothetic isotropic antenna
Hypothetic directional antenna
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Example
Source: NK Nikolova
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Equivalent half-‐power beamwidth representa8ons of an antenna’s radia8on paNern.
Volts
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Typical Gain and Beamwidth
Type of antenna Gi [dB] BeamW.
Isotropic 0 3600x3600
Half-wave Dipole 2 3600x1200
Helix (10 turn) 14 350x350
Small dish 16 300x300
Large dish 45 10x10
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Antenna Polariza8on
• The polariza8on of an antenna in a specific direc8on is defined to be the polariza8on of the wave produced by the antenna at a great distance at this direc8on
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Polariza8on states
450 LINEAR
UPPER HEMISPHERE: ELLIPTIC POLARIZATION LEFT_HANDED SENSE
LOWER HEMISPHERE: ELLIPTIC POLARIZATION RIGHT_HANDED SENSE
EQUATOR: LINEAR POLARIZATION
LATTITUDE: REPRESENTS AXIAL RATIO
LONGITUDE: REPRESENTS TILT ANGLE
POLES REPRESENT CIRCULAR POLARIZATIONS
LHC
RHC
(Poincaré sphere)
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Polariza8on filters/ reflectors
• At the surface of ideal conductor the tangen8al electrical field component = 0
|E1|>0 |E2| = 0
Vector E ⊥ wires Vector E || wires
|E1|>0 |E2| ~ |E2|
Wall of thin parallel wires (conductors)
Wire distance ~ 0.1λ
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Antenna arrays • Consist of mul8ple (usually iden8cal) antennas (elements) ‘collabora8ng’ to synthesize radia8on characteris8cs not available with a single antenna. They are able – to match the radia8on paNern to the desired coverage area – to change the radia8on paNern electronically (electronic scanning)
through the control of the phase and the amplitude of the signal fed to each element
– to adapt to changing signal condi8ons – to increase transmission capacity by beNer use of the radio resources
and reducing interference • Complex & costly
– Intensive research related to military, space, etc. ac8vi8es » Smart antennas, signal-‐processing antennas, tracking antennas, phased
arrays, etc.
Source: adapted from N Gregorieva
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2 GHz adap8ve antenna
• A set of 48 2GHz antennas – Source:
Arraycomm
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Phased Arrays
• Array of N antennas in a linear or two-‐dimensional configura8on + beam-‐forming & control device
• The amplitude and phase excita8on of each individual antenna controlled electronically (“sotware-‐defined”) – Diode phase shiters – Ferrite phase shiters
• Iner8a-‐less beam-‐forming and scanning (µsec) with fixed physical structure
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• Switched beam antennas – Based on switching func8on between
separate direc8ve antennas or predefined beams of an array
• Space Division Mul1ple Access (SDMA) = alloca8ng an angle direc8on sector to each user – In a TDMA system, two users will be
allocated to the same 8me slot and the same carrier frequency
– They will be differen8ated by different direc8on angles
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2 omnidirec8onal antennas
-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
D = 0.5λ, θ= 900
-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
D = 0.5λ, θ= 00 D = 0.5λ, θ= 1800
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Antenna Arrays: Benefits
• Possibili8es to control electronically – Direc8on of maximum radia8on – Direc8ons (posi8ons) of nulls – Beam-‐width – Direc8vity – Levels of sidelobes
using standard antennas (or antenna collec8ons) independently of their radia8on paNerns
• Antenna elements can be distributed along straight lines, arcs, squares, circles, etc.
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Antenna simulators • Polariza8on:
– hNp://www.amanogawa.com/archive/wavesA.html • Linear dipole antennas:
– hNp://www.amanogawa.com/archive/DipoleAnt/DipoleAnt-‐2.html – hNp://www.amanogawa.com/archive/Antenna1/Antenna1-‐2.html
• 2 antennas: – hNp://www.amanogawa.com/archive/TwoDipole/Antenna2-‐2.html
• Antenna design using MiniNEC – hNp://www.sotpedia.com/get/Science-‐CAD/Expert-‐MININEC-‐
Classic.shtml
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Thank you for your aNen8on