Transcript Document

School on Digital and Multimedia Communications Using Terrestrial and Satellite Radio Links
The Abdus Salam International Centre for Theoretical Physics ICTP Trieste (Italy) 12 February – 2 March 2001
Antenna Fundamentals (3)
R. Struzak
[email protected]
15 Feb 2001
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• Note: These materials may be used for study,
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please credit the author, Ryszard Struzak. These
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Ryszard Struzak. All commercial rights are
reserved. If you have comments or suggestions,
please contact the author at
[email protected].
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Summary Slide
•
•
•
•
Transmission vs. Reception
Polarization
More Complex Antennas
Antenna Arrays,
Adaptive Antennas
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Polarization
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Ex
Ey
Polarization ellipse
M

N
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• The two linear far-field
components radiated by
the horizontal and the
vertical antenna sum up to
a resultant elliptically
polarized wave
• The polarization ellipse is
defined by its axial ratio
N/M (ellipticity), tilt angle
 and sense of rotation
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Polarization states
LHC
UPPER HEMISPHERE:
ELLIPTIC POLARIZATION
LEFT_HANDED SENSE
(Poincaré sphere)
LATTITUDE:
REPRESENTS
AXIAL RATIO
EQUATOR:
LINEAR POLARIZATION
450 LINEAR
LOWER HEMISPHERE:
ELLIPTIC POLARIZATION
RIGHT_HANDED SENSE
RHC
LONGITUDE:
REPRESENTS
TILT ANGLE
POLES REPRESENT
CIRCULAR POLARIZATIONS
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Antenna Polarization
• The polarization of an antenna in a specific
direction is defined to be the polarization of the
wave produced by the antenna at a great distance
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Polarization Efficiency (1)
• The power received by an antenna
from a particular direction
is maximal if the polarization of the incident wave
has:
– the same axial ratio
– the same sense of polarization
– the same spatial orientation
as the polarization of the antenna in that direction.
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Polarization Efficiency (2)
• When the polarization of the incident wave is
different from the polarization of the receiving
antenna, then a loss due to polarization mismatch
occurs
Polarization efficiency =
= (power actually received) / (power that would be
received if the polarization of the incident wave
were matched to the receiving polarization of the
antenna)
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Polarization Efficiency (3)
LCH
A: POLARIZATION OF RECEIVING ANTENNA
W: POLARIZATION OF INCIDENT WAVE
W
2
A
Polarization
efficiency = cos2
450 LINEAR
H
RCH
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Circularly-Polarized Antenna
y
x
Ixcos(t+x)
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• Radio wave of any
polarization can be
Iycos(t+y)
obtained by superposition
of 2 linearly-polarized
waves produced by 2
crossed dipoles and by
controlling the amplituderatio and phase-difference
of their excitations.
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More Complex Antennas
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Antenna Over Ground: Image Theory
• Perfect ground = perfectly
conducting plane surface
• Tangential electrical field
component = 0
– vertical components: the
same direction
– horizontal components:
opposite directions
• The field (above the
ground) is the same if the
ground is replaced by the
antenna image
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-
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2 Antennas
• 2 identical antennas
r
– Excitation: I1 = I, I2 =Iej r
• Ant#1 field-strength:
rr

E’= C*D(, )
• Ant#2 field-strength:
2
E” = C*D(, )*ej(r+)
• E = E’ + E”
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d
1
r = d*cos 
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Antenna Array Factor (AAF)
• Resultant field-strength
E = E’ + E”
• E = C*D(, )*[1+ej(r+)]
= C*D(, )*AAF(, )  Pattern multiplication
• |AAF(, )|2 = Antenna array factor
= Gain of array of isotropic
antennas
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2 Antenna Array Factor (1)
• AAF() = 1+ej(r+) ;
(r+) = x
• AAF() = 1+ejx = 2[(1/2)(e-jx/2 +ejx/2)]ejx/2
= 2cos(x/2)ejx/2
• |AAF()| = 2cos(x/2)
= 2cos[(d/2)cos + /2)
= 2cos[(d/)cos + /2]
• |AAF()|2  Antenna Array Factor
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2 Antenna Array Factor (2)
• |AAF()|2 = {2cos[(d/)cos + /2]}2
• Gain: Max{|AAF()|2} = 4 (6 dBi)
when (d/)cos + /2 = 0, , …, k
• Nulls: when (d/)cos + /2 = /2, …, (k + 1)/2
• Relative gain = |AAF()|2 / Max{|AAF()|2}
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Demonstration (Simulation)
Array2ant
This program simulates radiation pattern
of 2 antenna-array factor.
It produces 2D diagrams showing
how the radiation lobes maximums
and minimums depends on the antennas
distance and excitation phases and magnitudes
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Antenna Arrays
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Yagi-Uda Arrays
• Only one antennaelement fed
• Other elements
unexcited (parasitic)
• Non-identical elements
• Non-identical distances
Directors
Reflector
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Driver
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Linear Array of n Antennas
• equally spaced
• F = 1+ejx+ej2x+ej3x+…+ej(N-1)x
antennas in line
= (1-ejNx) / (1-ejx)
• currents of equal
magnitude
• |F| = |(1-ejNx) / (1-ejx)|
• constant phase
= [sin(Nx/2) / sin(x/2)]
difference between
= F()  array factor
adjacent antennas
• numbered from 0
to (n-1)
• x/2 = (d/)cos + /2
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Demonstration (Simulation)
Array_Nan
This program simulates radiation pattern
of N - antenna-array factor.
It produces 2D diagrams showing
how the radiation lobes maximums
and minimums depends on the antenna
distance increment and on excitation phase and magnitude
functions
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Mutual Impedance
Array of antennas
V1 = I1Z11+I2Z12+…+InZ1n
V2 = I1Z12+I2Z22+…+InZ2n
.-……
Vn = I1Z1n+I2Z2n+…InZnn
Z1input = V1/I1= Z11+(I2/I1)Z12+…+(In/I1)Z1n
The input impedance depends on mutual impedance
(coupling) with other antennas and on relative currents
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Example: Impedance of Dipole
~73
/2
~300
</4
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Phased Arrays
• 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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Antenna Arrays: Benefits
• Possibilities to control
–
–
–
–
–
Direction of maximum radiation
Directions (positions) of nulls
Beam-width
Directivity
Levels of sidelobes
using standard antennas (or antenna collections)
independently of their radiation patterns
• Antenna elements can be distributed along straight
lines, arcs, squares, circles, etc.
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Beam Steering
Beam direction

d
3
2

• BeamEqui-phase
steering
wave front
using
phase
 = [(2/)d sin]
shifters at
Radiating
each
elements
radiating
Phase
0
shifters
element
Power
distribution
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4-Bit Phase-Shifter (Example)
Bit #3
Bit #4
Input
00
or
22.50
00
or
450
Bit #1
Bit #2
00
or
900
00
or
1800
Output
Steering/ Beam-forming Circuitry
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Switched-Line Phase Bit
Delay line
Input
Output
Diode switch
2 delay lines and 4 diodes per bit
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Switching Diode Circuit
PIN
diode
PIN
diode
Tuning
element
Tuning
element
b
a
a: RF short-circuited in forward bias
b: RF short-circuited in reverse bias
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Adaptive “Intelligent” Antennas
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Adaptive (“Intelligent”)Antennas
•
•
•
•
•
Array of N antennas in a linear
or spatial configuration
Used for receiving signals from
desired sources and suppress
incident signals from undesired
sources
The amplitude and phase
excitation of each individual
antenna controlled
electronically (“softwaredefined”)
The weight-determining
algorithm uses a-priori and/ or
measured information
The weight and summing
circuits can operate at the RF or
at an intermediate frequency
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w1

wN
N
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Weight-determining
algorithm
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