antena teory basic
TRANSCRIPT
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School on Digital Radio Communications for Research and Training in Developing Countries
The Abdus Salam International Centre for Theoretical Physics ICTP Trieste (Italy) 9 - 28 February 2004
Property of R. Struzak 1
Antenna theory basics
Prof. Dr. R. StruzakFormer Vice-Chairman, Radio Regulations Board, ITU
Note: These are preliminary notes, intended only for
distribution among the participants. Beware of misprints!
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Purpose
Property of R. Struzak 2
The purpose of the lecture is to refresh basicconcepts related to the antenna physics thatare needed to understand better the
operation and design of microwave digitalradio links
Note: Due to time limitations, we will focus here onlyon these aspects that are critical for microwave LAN/WAN applications.
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Main topics for discussion
Property of R. Struzak 3
Antenna concept & Reciprocity Theorem Point-antenna, Isotropic antenna
Antenna impedance, radiation pattern, &gain
E.I.R.P. and Power Transfer
Examples
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Intended & Unintended Antennas
Property of R. Struzak 4
Intended antennas Radiocommunication antennas
Measuring antennas, EM sensors, probes
EM applicators (Industrial, Medical)
Unintended antennas Any conductor/ installation carrying electrical current that vary in
time (e.g. electrical installation of vehicles) or any slot/ opening inthe device/ cable screen
Any conducting structure/ installation irradiated by EM waves Stationary (e.g. antenna masts or power line wires)
Time-varying (e.g. windmill or helicopter propellers)
Transient (e.g. aeroplanes, missiles)
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Antenna function
Property of R. Struzak 5
Transformation of a guided EM wave
in transmission line into a freely
propagating EM wave in space (or
vice versa)
From time-function in 1-D geometricalspace into time-function in 3-D space
(3-D geometrical space, polarization)
The specific form of the radiated waveis defined by the antenna structure and
its closest environment
Space wave
Guided wave
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Transition
Property of R. Struzak 6
If there is an inhomogeneity(obstacle) a reflected wave,standing wave, & higher fieldmodes appear
With pure standing wave theenergy is stored and oscillatesfrom entirely electric toentirely magnetic and back
Model: a resonator Q = (energy stored) / (energylost) per cycle, as in LC resonantLC circuits
Kraus p.2
Uniform wavetraveling
along the line
Transitionregion
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Dipole over plane
Property of R. Struzak 7
Broadband Thickwire
Smooth
NarrowbandThin
wire
Sharp
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3 functions
Property of R. Struzak 8
Antenna Must be of a size comparable with the wavelength to be
an efficient radiator
Transmission line Must be homogenous as much as possible to avoid
power reflections, otherwise use matching devices
Resonator For broadband applications resonances must be
attenuated
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Antenna impedance
Property of R. Struzak 9
RL
RR
jX - Energy stored in H, and E near-fieldcomponents; must be compensated
by matching devices
Energy radiated = transformed into heatin space (E & H, far-field components);
must be matched to the transmission line
Energy lost = transformed into heat in theantenna structure and nearby; must be
minimized
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Maxwells Equations
Property of R. Struzak 10
EM field interacting with the matter 2 coupled vectors E and H (6 numbers!), varying with
time and space and satisfying the boundary conditions
Reciprocity Theorem
Antenna characteristics do not depend on the directionof energy flow. The impedance & radiation pattern arethe same when the antenna radiates signal and when itreceives it (re-radiates).
Note: This theorem is valid only for linear passiveantennas (i.e. antennas that do not contain nonlinearand unilateral elements, e.g. amplifiers)
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EM Field of Current Element
Property of R. Struzak 11
HHHH
EEEE
r
r
++=++=
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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Short dipole antenna: summary
Property of R. Struzak 12
E &H are maximal in the equatorial plane, zero along the antenna
axis Eris maximal along the antenna axis dz, zero in the equatorial plane
All show axial symmetry
All are proportional to the current momentIdz
Have 3 components that decrease with the distance-to-wavelength ratioas (r/)-2 & (r/)-3: near-field, or induction field. The energy oscillates from
entirely electric to entirely magnetic and back, twice per cycle. Modeledas a resonant LC circuit or resonator;
(r/)-1: far-field or radiation field These 3 component are all equal at (r/) = 1/(2)
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Property of R. Struzak 13
Field components
0.001
0.01
0.1
1
10
100
1000
0.1 1 10
Relative distance, Br
Relativefie
ldstrength
FF
FF
Q
Q
C
C
FF: Radiation field
C, Q: Induction fields
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Field impedance
Property of R. Struzak 14
Field
impedanceZ = E/H
depends
on the
antenna
type andon
distance
0.01
0.1
1
10
100
0.01 0.1 1 10 100
Distance / (lambda/ 2Pi)
Z/377
Short dipole
Small loop
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Far-Field, Near-Field
Property of R. Struzak 15
Near-field region:
Angular distribution of energy depends ondistance from the antenna;
Reactive field components dominate (L, C)
Far-field region: Angular distribution of energy is independent on
distance;
Radiating field component dominates (R) The resultant EM field can locally be treated as
uniform (TEM)
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Poynting vector
Property of R. Struzak 16
The time-rate of EM energy flow per unit area in
free space is the Poynting vector.
It is the cross-product (vector product, right-hand
screw direction) of the electric field vector (E) andthe magnetic field vector (H): P = E x H.
For the elementary dipoleE H and onlyExHcarry energy into space with the speed of light.
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Power Flow
Property of R. Struzak 17
In free space and at large distances, the radiatedenergy streams from the antenna in radial lines,
i.e. the Poynting vector has only the radial
component in spherical coordinates. A source that radiates uniformly in all directions is
an isotropic source (radiator, antenna).
For such a source the radial component of thePoynting vector is independent of and .
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Linear Antennas
Property of R. Struzak 18
Summation of all vectorcomponents E (or H)
produced by each antennaelement
In the far-field region,
the vector componentsare parallel to each other
Method of moments
......
321
321
+++=+++=
HHHHEEEE
O
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Simulated experiment
Property of R. Struzak 19
http://www.amanogawa.com Linear dipole antenna java applet change the
dipole length
http://www.amanogawa.com/http://www.amanogawa.com/ -
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Planar (printed) antennas
Property of R. Struzak 20
FF
Slot
F
F
Slot & complementary antennas
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Reflector antenna
Property of R. Struzak 21
Intended reflectorantenna allowsmaintaining radiolink in non-LOS
conditions(avoiding
propagation
obstacles) Unintended antenna
create interference
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Reflector-type antennas
Property of R. Struzak 22
Reflector concentrates energy radiated Kraus p.324
Laboratory!
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The Green Bank Telescope
Property of R. Struzak 23
The largest
(140m), fullysteerable telescope
in the world
(Green Bank W.
Virginia). Its 8000ton surface points
to targets with
arcsecond
precision. (1998)
[Sky & Telescope,
Feb. 1997 p.28]
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The Arecibo Radio Telescope
Property of R. Struzak 24
The worlds
largest (1000-
foot) single
dish.
[Sky & Telescope Feb
1997 p. 29]
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Simple reflector antennas
Property of R. Struzak 25
Uda-Yagi, Log-periodic antennas
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Reflection & Image Theory
Property of R. Struzak 26
Antenna above perfectly
conducting plane surface Tangential electrical field
component = 0
vertical components: thesame direction
horizontal components:opposite directions
The field (above theground) is the same as ifthe ground is replaced bythe antenna image
+
-
Elliptical polarization:
change of the rotation sense!
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Lens-type antennas
Property of R. Struzak 27
Retardation of the
wave in dielectriclens Natural dielectric
Artificial dielectric
Acceleration of thewave in the metal-plate waveguide-type lens
Kraus p.382
Incidentwaveform
Transformed
waveform
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Aperture-antenna
Property of R. Struzak 28
EM wave
Power density: W [watt/square meter]
Effective aperture: [square meter]
A = P/W
Power absorbed: P [watt]
Note: The max effective aperture of linear/2wavelength dipole antenna is 2/8
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Horn antennas
Property of R. Struzak 29
Laboratory!
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Point Source
Property of R. Struzak 30
For many purposes, it is sufficient to knowthe direction (angle) variation of the powerradiated by antenna at large distances.
For that purpose, any practical antenna,regardless of its size and complexity, can berepresented as a point-source.
The actual field near the antenna is thendisregarded.
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Point Source 2
Property of R. Struzak 31
The EM field at large distances from an
antenna can be treated as originated at a
point source - fictitious volume-less emitter. The EM field in a homogenous unlimited
medium at large distances from an antennacan be approximated by an uniform plane
TEM wave
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Anisotropic sources
Property of R. Struzak 32
Every antenna has directional
properties (radiates more energy insome directions than in others).
Idealized example of directional
antenna: the radiated energy is
concentrated in the yellow region
(cone).
The power flux density gains: it is
increased by (roughly) the inverseratio of the yellow area and the total
surface of the isotropic sphere.
Isotropic sphere
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Antenna gain
Property of R. Struzak 33
Actual
antenna
P = Power
delivered to
the actual
antenna
S = Power
received
(the same in
both steps)
Measuring
equipment
Reference
antenna
Po = Power
delivered to
the reference
antenna
S0 = Power
received
(the same in
both steps)
Measuring
equipment
Antenna Gain = (P/Po) S=S0Step 1 Step 2
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Antenna Gains Gi, Gd, Gr
Property of R. Struzak 34
GiIsotropic Power Gain theoretical
concept, the reference antenna is isotropic
Gd - the reference antenna is a half-wave
dipole
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Antenna Gain: Comments
Property of R. Struzak 35
Unless otherwise specified, the gain refers
to the direction of maximum radiation.
Gain is a dimension-less factor related to
power and usually expressed in decibels
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Power vs. fieldstrength
Property of R. Struzak 36
Gain in the field
intensity may also
be considered - it is
equal to the squareroot of the power
gain.
2
00
2 2
0 377 ohms
for plane wavein free space
r r
E
P E P Z Z
E E E
Z
= =
= +
=
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e.i.r.p.
Property of R. Struzak 37
Equivalent Isotropically RadiatedPower (in a given direction):
The product of the power supplied to the
antenna and the antenna gain (relative to
an isotropic antenna) in a given direction
. . . . ie i r p PG=
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Antenna gain and effective area
Property of R. Struzak 38
Measure of the effective absorption area presented
by an antenna to an incident plane wave.
Depends on the antenna gain and wavelength
22( , ) [m ]
4eA G
=
Aperture efficiency: a = Ae / AA: physical area of antennas aperture, square meters
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Power Transfer in Free Space
Property of R. Struzak 39
: wavelength [m]
PR: power available at thereceiving antenna
PT: power delivered to the
transmitting antenna
GR: gain of the transmitting
antenna in the direction of the
receiving antenna
GT: gain of the receivingantenna in the direction of the
transmitting antenna
Matched polarizations
2
2
2
4
44
=
=
=
rGGP
G
r
PG
APFDP
RTT
RTT
eR
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Gain, Directivity, Radiation Efficiency
Property of R. Struzak 40
0
),(),(
P
P
DG
T=
=
The radiation intensity,directivity and gain are
measures of the ability of an
antenna to concentrate power
in a particular direction.
Directivity relates to the power
radiatedby antenna (P0
)
Gain relates to the power
delivered to antenna (PT)
: radiation efficiency(0.5 - 0.75)
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Directivity Pattern
Property of R. Struzak 41
The variation of the field intensity of an antenna as an
angular function with respect to the axis.
Usually represented graphically for the far-field
conditions.
May be considered for a specified polarization and/or
plane (horizontal, vertical).
Depends on the polarization and the reference plane for
which it is defined/measured
Synonym: Radiation pattern.
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Antenna patterns
Property of R. Struzak 42
P()/Pmax()
1
P()
Pmax()
Power pattern Relative (normalized)power pattern
Usually represented in 2 reference planes =const. and =const. E & PDF relate to the equivalent uniform plane wave
Note: Peak value = 2 x Effective value for sinusoidal quantities
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Elements of Radiation Pattern
Property of R. Struzak 43
0-180 180
Emax
Emax /2
Beamwidth
Sidelobes
Nulls
Main lobe Gain
Beam width
Nulls (positions)
Side-lobe levels
(envelope)
Front-to-back ratio
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Beam width
Property of R. Struzak 44
Beamwidth of an
antenna pattern: the angle
between the half-power
points of the main lobe.
Defined separately for
the horizontal plane andfor the vertical plane.
Usually expressed in
degrees.
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Typical Gain and Beamwidth
Property of R. Struzak 45
Type of antenna Gi [dB] BeamW.
Isotropic 0 3600x3600
Half-wave Dipole 2 3600x1200
Helix (10 turn) 14 350x350
Small dish 16 30
0
x30
0
Large dish 45 10x10
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Antenna Mask (Example 1)
Property of R. Struzak 46
Typical relative
directivity- mask
of receiving
antenna (Yagi
ant., TV dcm
waves)
[CCIR doc. 11/645, 17-Oct 1989)
-20
-15
-10
-5
0
-180
-120
-6 0 0
60
120
180
Azimith angle, degrees
Re
lative
gain,
dB
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Antenna Mask (Example 2)
Property of R. Struzak 47
-50
-40
-30
-20
-10
0
0.1 1 10 100
Phi/Phi0
Relative
gain
(dB)
RR/1998 APS30 Fig.9
COPOLAR
CROSSPOLAR
Reference pattern for co-polar and cross-polar components for satellite
transmitting antennas in Regions 1 and 3 (Broadcasting ~12 GHz)
0dB
-3dBPhi
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Property of R. Struzak 48
Linear Polarization
In a linearly polarized
plane wave the direction
of the E (or H) vector is
constant.
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Elliptical Polarization
Property of R. Struzak 49
LHC
Ex = cos (wt)
Ey = cos (wt+3pi/4)
Ex = cos (wt)
Ey = cos (wt+pi/4)Ex = cos (wt)
Ey = cos (wt)
Ex = cos (wt)
Ey = -sin (wt)
RHCEx = cos (wt)
Ey = sin (wt)Ex = cos (wt)
Ey = -cos (wt+pi/4)
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Property of R. Struzak 50
Polarization ellipse The superposition of
two plane-wavecomponents results inan elliptically
polarized wave The polarization
ellipse is defined by its
axial ratio N/M(ellipticity), tilt angle and sense of rotation
Ey
Ex
M
N
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Polarization states
Property of R. Struzak 51
LHC (Poincar sphere)UPPER HEMISPHERE:
ELLIPTIC POLARIZATIONLEFT_HANDED SENSE
EQUATOR:
LINEAR POLARIZATION
450 LINEARLOWER HEMISPHERE:ELLIPTIC POLARIZATION
RIGHT_HANDED SENSE
LATTITUDE:REPRESENTSAXIAL RATIO
LONGITUDE:REPRESENTSTILT ANGLE
POLES REPRESENTCIRCULAR POLARIZATIONS
RHC
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Comments on Polarization
Property of R. Struzak 52
At any moment in a chosen reference point in
space, there is actually a single electric vector E
(and associated magnetic vector H).
This is the result of superposition (addition) of theinstantaneous fields E (and H) produced by all
radiation sources active at the moment.
The separation of fields by their wavelength,polarization, or direction is the result of
filtration.
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Antenna Polarization
Property of R. Struzak 53
The polarization of an antenna in a specificdirection is defined to be the polarization of the
wave produced by the antenna at a great distance
at this direction
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Polarization Efficiency
Property of R. Struzak 54
The power received by an antenna
from a particular direction is maximal if the
polarization of the incident wave and the
polarization of the antenna in the wave arrival
direction have:
the same axial ratio
the same sense of polarization the same spatial orientation
.
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Polarization filters/ reflectors
Property of R. Struzak 55
Wall of thin parallel wires (conductors)
|E1|>0 |E1|>0|E2| = 0 |E2| ~ |E2|
Vector E wiresVector E || wires
Wire distance ~ 0.1
At the surface of ideal conductor the tangential
electrical field component = 0
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Phased Arrays
Property of R. Struzak 56
Array of N antennas in a linear or spatial
configuration The amplitude and phase excitation of each
individual antenna controlled electronically
(software-defined) Diode phase shifters
Ferrite phase shifters
Inertia-less beam-forming and scanning (sec)with fixed physical structure
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Beam Steering
Property of R. Struzak 57
Beam-
steeringusing
phase
shifters ateach
radiating
element
Radiating
elements
Power
distribution
Phase
shifters
Equi-phasewave front
= [(2/)d sin]
3 2 0
d
Beam direction
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4-Bit Phase-Shifter (Example)
Property of R. Struzak 58
Bit #1Bit #3Bit #4 Bit #2
Input Output00 or22.50 00 or450 00 or900 00 or1800
Steering/ Beam-forming Circuitry
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Switched-Line Phase Bit
Property of R. Struzak 59
Input Output
Delay line #1
Delay line #2
Diode switch
Phase bit = delay difference
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Delay line w. nonlinear dielectric
Property of R. Struzak 60
LZ
C
=
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2 omnidirectional antennas
Property of R. Struzak 61
-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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N omnidirectional antennas
Property of R. Struzak 62
0
0.5
1
1.5
2
2.5
-180 -90 0 90 180
Azimuth angle, degrees
Relativegai
0
1
2
3
4
5
6
-180 -90 0 90 180
Azimuth angle, degrees
Relativegain
0
1
2
3
4
5
6
7
8
9
10
-180 -90 0 90 180
Azimuth angle, degrees
Relativegai
N = 9, = 450N = 2, = 900 N = 5, = 1800
Array gain (line, uniform, identical power)
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Antenna synthesis objective
Property of R. Struzak 63
Main lobe looks in the
required direction
The signal delivered to N
fixed points is above therequired minimal level
The signal delivered to N
fixed points is zero, orbelow the required
maximal level
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Owens Valley Radio Observatory
Property of R. Struzak 64
The Earths
atmosphere is
transparent inthe narrow
visible-light
window
(4000-7000
angstroms) and
the radio band
between 1 mm
and 10 m.
[Sky & Telescope
Feb 1997 p.26]
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The New Mexico Very Large Array
Property of R. Struzak 65
[Sky & Telescope
Feb 1997 p. 30]
27 antennas along 3 railroad tracks provide baselines up to 35 km. Radio
images are formed by correlating the signals garnered by each antenna.
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Circular antenna arrays
Property of R. Struzak 66
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Antenna Arrays: Benefits
Property of R. Struzak 67
Possibilities to control electronically
Direction of maximum radiation
Directions (positions) of nulls
Beam-width
Directivity
Levels of sidelobesusing standard antennas (or antenna collections)
independently of their radiation patterns
Antenna elements can be distributed along straightlines, arcs, squares, circles, etc.
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Adaptive (Intelligent)Antennas
Property of R. Struzak 68
Array of N antennas in a linearor spatial configuration
Used for selection receiving
signals from desired sources andsuppress incident signals fromundesired sources
The amplitude and phaseexcitation of each individual
antenna controlledelectronically (software-defined)
The weight-determiningalgorithm uses a-priori and/ or
measured information The weight and summingcircuits can operate at the RF orat an intermediate frequency
w1
wN
Weight-determiningalgorithm
1
N
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Protocol independence
Property of R. Struzak 69
Transceiver
Channelizer
(TDMA, FDMA, CDMA)
antenna antenna
Transceiver
Channelizer
(TDMA, FDMA, CDMA)
Spatial and Temporal Processing
baseband signals/user data
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2 GHz adaptive antenna
Property of R. Struzak 70
A set of 48
2GHz
antennas
Source:Arraycomm
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Simulated experiments
Property of R. Struzak 71
2 omnidirectional antennas (equal amplitudes)
N omnidirectional antennas (arranged along a
line, with equal amplitudes)
Variables (In both cases):
distance increment
phase increment
http://array2ant.xls/http://array_nant.xls/http://array_nant.xls/http://array2ant.xls/ -
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What we have learned
Property of R. Struzak 72
Antenna: substantial element of radio link
We have just reviewed
Basic concepts
Radio wave radiation physics Elementary radiators
Selected issues relevant to antennas
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Selected References
Property of R. Struzak 73
Griffiths H & Smith BL (ed.):Modern antennas; Chapman & Hall, 1998
Johnson RC:Antenna Engineering Handbook McGraw-Hill Book Co.
1993 Kraus JD:Antennas, McGraw-Hill Book Co. 1998
Scoughton TE:Antenna Basics Tutorial; Microwave Journal Jan. 1998, p.186-191
Stutzman WL, Thiele GA:Antenna Theory and Design JWiley &Sons,1981
http://amanogawa.com Software
http://www.feko.co.za/apl_ant_pla.htm Li et al., Microcomputer Tools for Communication Engineering Pozar D. Antenna Design Using Personal Computers www.gsl.net/wb6tpu /swindex.html (NEC Archives)
http://amanogawa.com/http://www.feko.co.za/apl_ant_pla.htmhttp://www.feko.co.za/apl_ant_pla.htmhttp://www.gsl.net/wb6tpu%20/swindex.htmlhttp://www.gsl.net/wb6tpu%20/swindex.htmlhttp://www.gsl.net/wb6tpu%20/swindex.htmlhttp://www.gsl.net/wb6tpu%20/swindex.htmlhttp://www.feko.co.za/apl_ant_pla.htmhttp://amanogawa.com/ -
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