antennas and radiating systems.pdf
TRANSCRIPT
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Radiation of electromagnetic waves
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Outline
z Introduction
zReview of basic antenna typeszRadiation pattern, gain, polarization
zEquivalent circuit & radiation efficiencyzSmart antennas
zSome theoryzSummary
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Antenna purpose
z Transformation of a guided EM
wave in transmission line(waveguide) into a freelypropagating EM wave in space (orvice versa) with specifieddirectional characteristics
{Transformation from time-function inone-dimensional space into time-
function in three dimensional space{The specific form of the radiated
wave is defined by the antennastructure and the environment
Space wave
Guided wave
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Antenna functions
zTransmission line{Power transport medium - must avoid power
reflections, otherwise use matching devices
zRadiator{Must radiate efficiently must be of a size
comparable with the half-wavelength
zResonator{Unavoidable - for broadband applications
resonances must be attenuated
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Monopole (dipole over plane)
Low-QBroadband
High-QNarrowband
z If there is an inhomogeneity (obstacle) a reflected wave, standing wave, & higher field modesappear
z With pure standing wave the energy is stored and oscillates from entirely electric to entirelymagnetic and back
z Model: a resonator with high Q = (energy stored) / (energy lost) per cycle, as in LC circuitsz Kraus p.2
Smooth
transitionregion
Uniform wave
travelingalong the line
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Outline
z Introduction
zReview of basic antenna typeszRadiation pattern, gain, polarization
zEquivalent circuit & radiation efficiencyzSmart antennas
zSome theoryzSummary
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Antennas for laptop applications
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
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z Patch and slot antennasderived from printed-circuit andmicro-strip technologies
z Ceramic chip antennas aretypically helical or inverted-F(INF) antennas, or variations ofthese two types with high
dielectric loading to reduce theantenna size
Source: D. Liu et al.: Developing integrated antenna subsystems for laptopcomputers; IBM J. RES. & DEV. VOL. 47 NO. 2/3 MARCH/MAY 2003 p. 355-367
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Slot & INF antennasz Slot antenna: a slot is cut from a large (relative
to the slot length) metal plate.zThe center conductor of the feeding coaxial cable is
connected to one side of the slot, and the outsideconductor of the cable - to the other side of the slot.
z The slot length is some (/2) for the slot antennaand (/4) long for the INF antenna.
z The slot and INF antennas behave similarly.zThe slot antenna can be considered as a loaded version
of the INF antenna. The load is a quarter-wavelength stub,
i.e. a narrowband device.zWhen the feed point is moved to the short-circuited end ofthe slot (or INF) antenna, the impedance decreases.When it is moved to the slot center (or open end of theINF antenna), the impedance increases
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Exampledouble-layer printed Yagi antenna
Source: N Gregorieva
Note: no galvanic contact with thedirector
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zPatch and slot antennas are
{Cheap and easy to fabricate and to mount{Suited for integration
{Light and mechanically robust
{Have low cross-polarization
{Low-profile - widely used in antenna arrays
spacecrafts, satellites, missiles, cars and other mobileapplications
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Aperture-antenna
Note: The aperture concept is applicablealso to wired antennas. For instance,the max effective aperture of linear/2 wavelength dipole antenna is 2/8
EM wave
Powerabsorbed: P [watt]
Power density:PFD [w/m2]
Effectiveaperture: A[m2]
A = A*PFD
z Aperture antennasderived fromwaveguide technology(circular, rectangular)
z Can transfer high
power (magnetrons,klystrons)
z Above few GHz
z Will be exploredinprace during theschool
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Leaky-wave antennas
z Derived from millimeter-wave guides (dielectric
guides, microstrip lines,coplanar and slot lines).
z For frequencies > 30 GHz,
including infraredz Subject of intensive study.
{ Note: Periodicaldiscontinuities near the end
of the guide lead tosubstantial radiationleakage (radiation from thedielectric surface).
Source: adapted from N Gregorieva
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Reflector antennas
zReflectors are used to concentrate flux of EMenergy radiated/ received, or to change itsdirection
zUsually, they are parabolic (paraboloidal).{
The first parabolic (cylinder) reflector antenna was usedby Heinrich Hertz in 1888.
z Large reflectors have high gain and directivity{Are not easy to fabricate
{Are not mechanically robust
{Typical applications: radio telescopes, satellitetelecommunications.
Source: adapted from N Gregorieva
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Planar reflectors
z Uda-Yagi, Log-periodic antennas
d
2d
z Intended reflector antennaallows maintaining radio link in
non-LOS conditions (avoidingpropagation obstacles)
z Unintended antennas createinterference
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Paraboloidal reflectors
Front feed Cassegrain feed
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The largest radio telescopes
Max Plank Institt fr Radioastronomie
radio telescope, Effelsberg (Germany),100-m paraboloidal reflector
zThe Green Bank Telescope (the NationalRadio Astronomy Observatory) paraboloid of aperture 100 m
Source: adapted from N Gregorieva
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The Arecibo Observatory Antenna
SystemThe worldslargest single
radio telescope
304.8-msphericalreflector
NationalAstronomy andIonosphereCenter (USA),Arecibo,Puerto Rico
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The Arecibo Radio Telescope
[Sky & TelescopeFeb 1997 p. 29]
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Lens antennas
Source: Kraus p.382, N Gregorieva
Lenses play a similar role to that of reflectors in reflector antennas:they collimate divergent energyOften preferred to reflectors at frequencies > 100 GHz.
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Outline
z Introduction
zReview of basic antenna typeszRadiation pattern, gain, polarization
zEquivalent circuit & radiation efficiencyzSmart antennas
zSome theoryzSummary
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Radiation pattern
z The radiation pattern of antenna is a representation(pictorial or mathematical) of the distribution of the power
out-flowing (radiated) from the antenna (in the case oftransmitting antenna), or inflowing (received) to theantenna (in the case of receiving antenna) as a function
of direction angles from the antennazAntenna radiation pattern (antenna pattern):
is defined for large distances from the antenna, where the spatial(angular) distribution of the radiated power does not depend on the
distance from the radiation source is independent on the power flow direction: it is the same when the
antenna is used to transmit and when it is used to receive radio waves
is usually different for different frequencies and different polarizations
of radio wave radiated/ received
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Power pattern vs. Field pattern
z The power pattern is themeasured (calculated)
and plotted receivedpower: |P(, )| at aconstant (large) distancefrom the antenna
z The amplitude fieldpattern is the measured(calculated) and plottedelectric (magnetic) field
intensity, |E(
, )| or|H(, )| at a constant(large) distance from theantenna
Power orfield-strength meter
Antennaunder test
Turntable
Generator
Auxiliaryantenna
Large distance
z The power pattern and the field
patterns are inter-related:P(, ) = (1/)*|E(, )|2 = *|H(,)|2
P = power
E = electrical field component vector
H = magnetic field component vector
= 377 ohm (free-space, plane wave
impedance)
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Normalized pattern
zUsually, the pattern describes the
normalized field (power) values withrespect to the maximum value.
{Note: The power pattern and the amplitude fieldpattern are the same when computed and whenplotted in dB.
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3-D pattern
z Antenna radiation
pattern is3-dimensional
z The 3-D plot of antenna
pattern assumes bothangles and varying,which is difficult toproduce and to interpret
3-D pattern
Source: NK Nikolova
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2-D pattern
Two 2-D patterns
z Usually the antenna
pattern is presented as a2-D plot, with only one ofthe direction angles, or varies
zIt is an intersection of the3-D one with a given plane{ usually it is a = const
plane or a = const planethat contains the patternsmaximum
Source: NK Nikolova
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Example: a short dipole on z-axis
Source: NK Nikolova
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Principal patterns
z Principal patterns are the 2-D patterns
of linearly polarized antennas,measured in 2 planes
1. the E-plane: a plane parallel to the Evector and containing the direction ofmaximum radiation, and
2. the H-plane: a plane parallel to the Hvector, orthogonal to the E-plane, andcontaining the direction of maximum
radiation Source: NK Nikolova
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Example
Source: NK Nikolova
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Antenna Mask (Example 1)
Typical relative
directivity- maskof receivingantenna (Yagiant., TV dcmwaves)
[CCIR doc. 11/645, 17-Oct 1989)
-20
-15
-10
-5
0
-180
-120
-60 0
60
120
180
Azimi th angle, degrees
Relativegain,
dB
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Antenna Mask (Example 2)
-50
-40
-30
-20
-10
0
0.1 1 10 100
Phi/Phi0
Rela
tive
gain
(dB)
RR/1998 APS30 Fig.9
COPOLAR
CROSSPOLAR
Reference pattern for co-polar and cross-polar components forsatellite transmitting antennas in Regions 1 and 3 (Broadcasting
~12 GHz)
0dB
-3dBPhi
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Isotropic antenna
z Isotropic antenna orisotropic radiatoris a
hypothetical (not physicallyrealizable) concept, used as auseful reference to describe
real antennas.z Isotropic antenna radiatesequally in all directions.{Its radiation pattern is
represented by a sphere whosecenter coincides with the locationof the isotropic radiator.
Source: NK Nikolova
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Directional antenna
zDirectional antenna is an antenna, which
radiates (or receives) much more power in(or from) some directions than in (or from)others.
{Note: Usually, this term is applied to antennaswhose directivity is much higher than that of a
half-wavelength dipole.
Source: NK Nikolova
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Omnidirectional antenna
zAn antenna, which
has a non-directional patternin a plane
{It is usuallydirectional in other
planes
Source: NK Nikolova
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Pattern lobes
Source: NK Nikolova
Pattern lobe is a
portion of the radiationpattern with a localmaximum
Lobes are classifiedas: major, minor,side lobes, backlobes.
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Pattern lobes and beam widths
Source: NK Nikolova
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Beamwidth
zHalf-power beamwidth (HPBW) is the anglebetween two vectors from the patterns origin tothe points of the major lobe where the radiationintensity is half its maximum
zOften used to describe the antenna resolution properties
y Important in radar technology, radioastronomy, etc.
z First-null beamwidth (FNBW) is the angle
between two vectors, originating at the patternsorigin and tangent to the main beam at its base.y Often FNBW 2*HPBW
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Example
Source: NK Nikolova
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Anisotropic sources: gain
z Every real antenna radiates moreenergy in some directions than inothers (i.e. has directional properties)
z Idealized example of directionalantenna: the radiated energy isconcentrated in the yellow region(cone).
z Directive antenna gain: the power fluxdensity is increased by (roughly) theinverse ratio of the yellow area and thetotal surface of the isotropic sphere
{ Gain in the field intensity may also beconsidered - it is equal to the squareroot of the power gain.
Isotropic sphere
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Antenna gain measurement
Antenna Gain = (P/Po) S=S0
Actualantenna
P = Powerdelivered tothe actualantenna
S = Powerreceived
(the same inboth steps)
Measuringequipment
Step 2: substitution
Referenceantenna
Po = Powerdelivered tothe reference
antenna
S0 = Powerreceived
(the same inboth steps)
Measuringequipment
Step 1: reference
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Antenna Gains Gi, Gd
zUnless otherwise specified, the gain refers
to the direction of maximum radiation.zGain is a dimension-less factor related to
power and usually expressed in decibels
zGi Isotropic Power Gain theoreticalconcept, the reference antenna is isotropic
zGd - the reference antenna is a half-wavedipole
T i l G i d B idth
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Typical Gain and Beamwidth
Type of antenna Gi [dB] BeamW.
Isotropic 0 3600x3600
Half-wave Dipole 2 360
0
x1200
Helix (10 turn) 14 350x350
Small dish 16 300x300
Large dish 45 10x10
A t i d ff ti
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Antenna gain and effective area
zMeasure of the effective absorption areapresented by an antenna to an incident planewave.
zDepends on the antenna gain and wavelength2 2( , ) [m ]4
eA G
=
Aperture efficiency: a = Ae / AA: physical area of antennas aperture, square meters
P T f i F S
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Power Transfer in Free Space
z : wavelength [m]
z PR: power available at the
receiving antennaz PT: power delivered to the
transmitting antenna
z GR: gain of the transmitting
antenna in the direction ofthe receiving antenna
z GT: gain of the receivingantenna in the direction ofthe transmitting antenna
z Matched polarizations
2
2
2
4
44
=
=
=
rGGP
G
r
PG
APFDP
RTT
RTT
eR
i
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e.i.r.p.
zEquivalent Isotropically Radiated
Power (in a given direction):
zThe product of the power supplied to the
antenna and the antenna gain (relativeto an isotropic antenna) in a givendirection
. . . . ie i r p PG=
Li P l i ti
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Linear Polarization
z In a linearly polarizedplane wave the directionof the E (or H) vector is
constant.
z http://www.amanogawa.com/archive/wavesA.h
tml
Elliptical Polari ation
http://www.amanogawa.com/archive/wavesA.htmlhttp://www.amanogawa.com/archive/wavesA.htmlhttp://www.amanogawa.com/archive/wavesA.htmlhttp://www.amanogawa.com/archive/wavesA.html -
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Elliptical Polarization
Ex = cos (wt)
Ey = cos (wt)
Ex = cos (wt)
Ey = cos (wt+pi/4)Ex = cos (wt)
Ey = -sin (wt)
Ex = cos (wt)
Ey = cos (wt+3pi/4)
Ex = cos (wt)
Ey = -cos (wt+pi/4)
Ex = cos (wt)
Ey = sin (wt)
LHC
RHC
Polarization ellipse
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Polarization ellipse
z The superposition oftwo plane-wave
components results inan ellipticallypolarized wave
z The polarizationellipse is defined byits axial ratio N/M
(ellipticity), tilt angle and sense of rotation
Ey
Ex
M
N
Polarization states
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Polarization states
450 LINEAR
UPPER HEMISPHERE:
ELLIPTIC POLARIZATIONLEFT_HANDED SENSE
LOWER HEMISPHERE:ELLIPTIC POLARIZATION
RIGHT_HANDED SENSE
EQUATOR:
LINEAR POLARIZATION
LATTITUDE:REPRESENTSAXIAL RATIO
LONGITUDE:REPRESENTSTILT ANGLE
POLES REPRESENTCIRCULAR POLARIZATIONS
LHC
RHC
(Poincar sphere)
Comments on Polarization
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Comments on Polarization
zAt any moment in a chosen reference point inspace, there is actually a single electric vector E(and associated magnetic vector H).
z This is the result of superposition (addition) of
the instantaneous fields E (and H) produced byall radiation sources active at the moment.
z The separation of fields by their wavelength,
polarization, or direction is the result of filtration.
Antenna Polarization
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Antenna Polarization
z The polarization of an antenna in a specificdirection is defined to be the polarization of thewave produced by the antenna at a great
distance at this direction
Polarization Efficiency
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Polarization Efficiency
z The power received by an antenna
from a particular direction is maximal if thepolarization of the incident wave and thepolarization of the antenna in the wave arrivaldirection have:{ the same axial ratio
{the same sense of polarization
{ the same spatial orientation
.
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Power transfer
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Power transfer
zThe maximumpower is deliveredto (or from) theantenna when the
antennaimpedance and theimpedance of the
equivalentgenerator (or load)are matched
0
0.5
1
0.1 1 10
RA / RG; (XA+XG = 0)
P
A
/PAmax
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zWhen the antenna impedance is not matched tothe transmitter output impedance (or to the
receiver input impedance) or to the transmissionline between them, impedance-matchingdevices must be used for maximum power
transferz Inexpensive impedance-matching devices are
usually narrow-band
z Transmission lines often have significant losses
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Antenna arrays
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y
z Consist of multiple (usually identical) antennas (elements)collaborating to synthesize radiation characteristics notavailable with a single antenna. They are able{ to match the radiation pattern to the desired coverage area{ to change the radiation pattern electronically (electronic
scanning) through the control of the phase and the amplitude ofthe signal fed to each element
{ to adapt to changing signal conditions{ to increase transmission capacity by better use of the radio
resources and reducing interference
z Complex & costly Intensive research related to military, space, etc. activities
y Smart antennas, signal-processing antennas, tracking antennas,phased arrays, etc.
Source: adapted from N Gregorieva
Satellite antennas (TV)
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( )
zNot an array!
Owens Valley Radio Observatory
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y y
The Earthsatmosphere istransparent in
the narrowvisible-lightwindow(4000-7000angstroms) and
the radio bandbetween 1 mmand 10 m.
[Sky & Telescope
Feb 1997 p.26]
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The New Mexico Very Large Array
27 antennas along 3 railroad tracks provide baselines up to 35 km.
Radio images are formed by correlating the signals garnered byeach antenna.
[Sky & TelescopeFeb 1997 p. 30]
2 GHz adaptive antenna
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p
zA set of 48
2GHzantennas{ Source:
Arraycomm
Phased Arrays
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y
zArray of N antennas in a linear or two-dimensional configuration + beam-forming
& control devicez The amplitude and phase excitation of each
individual antenna controlled electronically
(software-defined){Diode phase shifters
{Ferrite phase shifters
z Inertia-less beam-forming and scanning (sec)with fixed physical structure
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zDynamically phased array
(PA):{A generalization of the
switched lobe concept
{The radiation patterncontinuously track thedesignated signal (user)
{Include a direction of arrival(DoA) tracking algorithm
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Switched-Line Phase Bit
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Phase bit = delay difference
Input Output
Diode switch
Delay line #1a
Delay line #1b
Simulation
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z 2 omnidirectional antennas (equal amplitudes)
{Variableszdistance increment
zphase increment
zN omnidirectional antennas{Group factor (N omnidirectional antennas uniformly
distributed along a straight line, equal amplitudes, equalphase increment)
z http://www.amanogawa.com/archive/TwoDipole/Antenna2-2.html (more details)
2 omnidirectional antennas
http://array2ant.xls/http://array_nant.xls/http://www.amanogawa.com/archive/TwoDipole/Antenna2-2.htmlhttp://www.amanogawa.com/archive/TwoDipole/Antenna2-2.htmlhttp://www.amanogawa.com/archive/TwoDipole/Antenna2-2.htmlhttp://www.amanogawa.com/archive/TwoDipole/Antenna2-2.htmlhttp://array_nant.xls/http://array2ant.xls/ -
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-1
-0.5
0
0.5
1
-1 -0.5 0 0.5 1
D = 0.5, = 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 = 0.5, = 00 D = 0.5, = 1800
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Adaptive (Intelligent)Antennas
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z Array of N antennas in a linear,circular, or planar configuration
z Used for selection signals from
desired sources and suppressincident signals from undesiredsources
z The antenna pattern track the
sourcesz It is then adjusted to null out the
interferers and to maximize thesignal to interference ratio (SIR)
z Able to receive and combineconstructively multipath signals
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EM Field of Current Element
HHHH
EEEE
r
r
GGGG
GGGK
++=
++=
I: current (monochromatic) [A]; dz: antenna element (short) [m]x
y
z
OP
r
ErE
E
I, dz222
222
HHHH
EEEE
r
r
++=
++=
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Linear Antennas
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z Summation of all vectorcomponents E (or H)produced by each antenna
element
z In the far-field region,the vector componentsare parallel to each other
z Phase difference due to{ Excitation phase difference{ Path distance difference
z Method of moments
...
...
321
321
+++=
+++=
HHHH
EEEEGGGG
GGGK
O
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Antenna Bandwidth
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Zeland Software, Inc. & BAY TECHNOLOGY,
IE3D Benchmark Simulation Example
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Far Field Expression
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hI
R
ejkE
jkR
4~
R
Vg
ERH
~
Antenna Pattern 3D
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Antenna Simulation
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Zeland Software, Inc. & BAY TECHNOLOGY, IE3D Benchmark Simulation Example
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Equivalent Circuit ofReceiving Antenna
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Voc
Z A
Z LZo
hEVi
oc =
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Effective Area of aReceiving Antenna
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D
AA
e
e
i
A
i
A
i
r
GR
h
R
hA
AE
R
hE
R
hEP
444
2428
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2
22
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22222
=
==
=
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Input Impedance
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,,, 1992
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Image Theory forElectric Currents
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Image Theory forMagnetic Currents
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Microstrip Traveling-Wave Antennas
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,,, 1992
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Microstrip Antenna Array Element
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Antenna Array
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