15 Feb 2001Property of R. Struzak1 Antenna Fundamentals (3) R. Struzak [email protected]...
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Transcript of 15 Feb 2001Property of R. Struzak1 Antenna Fundamentals (3) R. Struzak [email protected]...
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15 Feb 2001 Property of R. Struzak 1
Antenna Fundamentals (3)
School on Digital and Multimedia Communications Using Terrestrial and Satellite Radio LinksThe Abdus Salam International Centre for Theoretical Physics ICTP Trieste (Italy) 12 February – 2 March 2001
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15 Feb 2001 Property of R. Struzak 2
• Note: These materials may be used for study, research, and education in not-for-profit applications. If you link to or cite these materials, please credit the author, Ryszard Struzak. These materials may not be published, copied to or issued from another Web server without the author's express permission. Copyright © 2001 Ryszard Struzak. All commercial rights are reserved. If you have comments or suggestions, please contact the author at [email protected].
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15 Feb 2001 Property of R. Struzak 3
Summary Slide
• Transmission vs. Reception
• Polarization
• More Complex Antennas
• Antenna Arrays, Adaptive Antennas
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15 Feb 2001 Property of R. Struzak 4
Polarization
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15 Feb 2001 Property of R. Struzak 5
Polarization ellipse
• 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
Ey
Ex
M
N
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15 Feb 2001 Property of R. Struzak 6
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
(Poincaré sphere)
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15 Feb 2001 Property of R. Struzak 7
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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15 Feb 2001 Property of R. Struzak 8
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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15 Feb 2001 Property of R. Struzak 9
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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15 Feb 2001 Property of R. Struzak 10
Polarization Efficiency (3)
H
RCH
LCH
450 LINEAR
2 Polarization efficiency = cos2
W
A
A: POLARIZATION OF RECEIVING ANTENNA W: POLARIZATION OF INCIDENT WAVE
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15 Feb 2001 Property of R. Struzak 11
Circularly-Polarized Antenna
• Radio wave of any polarization can be obtained by superposition of 2 linearly-polarized waves produced by 2 crossed dipoles and by controlling the amplitude- ratio and phase-difference of their excitations.
y
x
Ixcos(t+x)
Iycos(t+y)
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15 Feb 2001 Property of R. Struzak 12
More Complex Antennas
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15 Feb 2001 Property of R. Struzak 13
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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15 Feb 2001 Property of R. Struzak 14
2 Antennas
• 2 identical antennas– Excitation: I1 = I, I2 =Iej
• Ant#1 field-strength: E’= C*D(, )
• Ant#2 field-strength:E” = C*D(, )*ej(r+)
• E = E’ + E”
r = d*cos
12
rr
r
r
d
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15 Feb 2001 Property of R. Struzak 15
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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15 Feb 2001 Property of R. Struzak 16
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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15 Feb 2001 Property of R. Struzak 17
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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15 Feb 2001 Property of R. Struzak 18
Demonstration (Simulation)
Array2antThis 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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15 Feb 2001 Property of R. Struzak 19
Antenna Arrays
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15 Feb 2001 Property of R. Struzak 20
Yagi-Uda Arrays
• Only one antenna- element fed
• Other elements unexcited (parasitic)
• Non-identical elements• Non-identical distances
Directors
Reflector Driver
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15 Feb 2001 Property of R. Struzak 21
Linear Array of n Antennas
• equally spaced antennas in line
• currents of equal magnitude
• constant phase difference between adjacent antennas
• numbered from 0 to (n-1)
• F = 1+ejx+ej2x+ej3x+…+ej(N-1)x
= (1-ejNx) / (1-ejx)
• |F| = |(1-ejNx) / (1-ejx)| = [sin(Nx/2) / sin(x/2)] = F() array factor
• x/2 = (d/)cos + /2
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15 Feb 2001 Property of R. Struzak 22
Demonstration (Simulation)
Array_NanThis 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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15 Feb 2001 Property of R. Struzak 23
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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15 Feb 2001 Property of R. Struzak 24
Example: Impedance of Dipole
~73 ~300/2
</4
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15 Feb 2001 Property of R. Struzak 25
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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15 Feb 2001 Property of R. Struzak 26
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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15 Feb 2001 Property of R. Struzak 27
Beam Steering
• Beam-steering using phase shifters at each radiating element
Radiatingelements
Powerdistribution
Phaseshifters
Equi-phasewave front
= [(2/)d sin]
3 2 0
d
Beam direction
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15 Feb 2001 Property of R. Struzak 28
4-Bit Phase-Shifter (Example)
00 or 22.50 00 or 450 00 or 900 00 or 1800Input Output
Bit #4 Bit #3 Bit #2 Bit #1
Steering/ Beam-forming Circuitry
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15 Feb 2001 Property of R. Struzak 29
Switched-Line Phase Bit
2 delay lines and 4 diodes per bit
Input Output
Diode switch
Delay line
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15 Feb 2001 Property of R. Struzak 30
Switching Diode Circuit
a: RF short-circuited in forward biasb: RF short-circuited in reverse bias
PINdiode
Tuningelement
PINdiode
Tuningelement
a b
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15 Feb 2001 Property of R. Struzak 31
Adaptive “Intelligent” Antennas
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15 Feb 2001 Property of R. Struzak 32
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 (“software-defined”)
• 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
w1
wN
Weight-determining algorithm
1
N