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Notes 17
Polarization of Plane Waves
1
ECE 3317 Applied Electromagnetic Waves
Prof. David R. Jackson Fall 2020
Polarization
The polarization of a plane wave refers to the direction of the electric field vector in the time domain.
We assume that the wave is traveling in the positive z direction.
2
( , )z tE
( ),z tS
x
y
z
Consider a plane wave with both x and y components
ˆ ˆ( ) ( ) jkzx yE z x E y E e−= +
( = )y xE Eβ −In general, phase of phase of
Assume: xj
y
E aE be β
= =
=
real number
Polarization (cont.)
Phasor domain:
3
x
y
xE
yE
Time Domain:
( ) ( )
( ) ( )
Re cos
Re cos
j tx
j j ty
ae a t
be e b t
ω
β ω
ω
ω β
= =
= = +
E
E
Depending on b/a and β, three different cases arise:
4
Linear polarization Circular polarization Elliptical polarization
Polarization (cont.)
x
y
( )tE
0z =
Power Density: *12
S E H= ×
( )*
1 ˆ ˆ ˆ ˆ2
y xx y
E ES x E y E x yη η
= + × − +
, yxy x
EEH Hη η
= = −
( )221 ˆ2 x yS z E Eη
= +
Assume lossless medium (η is real):
5
21 ˆ2
S z Eη
=or
From Faraday’s law:
Polarization (cont.)
Hence
At z = 0:
( )ˆ ˆ cosx a y b tω= ±E
coscos
x
y
a tb t
ωω
== ±
EE
Linear Polarization
6
( )( )
cos
cosx
y
a t
b t
ω
ω β
=
= +
E
E
0β β π= =or
0ββ π
+ =− =
sign :sign :
(shown for β = 0)
Linear Polarization (cont.)
7
This is simply a “tilted” plane wave.
( )ˆ ˆ cosx a y b tω= ±E
( )tE
x
y x′y′
a
b
φ
1tan ba
φ − = ±
At z = 0:
2 2 2 2 2 2 2 2cos sinx y a t a t aω ω= + = + =E E E
coscos( / 2) sin
x
y
a ta t a t
ωω π ω
== ± =
EE
Circular Polarization
8
/ 2b a β π= = ±and
( )( )
cos
cosx
y
a t
b t
ω
ω β
=
= +
E
E
Note: The top sign is always for β = +π /2.
( ) 1 1tan tan ( tan )y
x
t tφ ω− − = =
EE
( )t tφ ω=
Circular Polarization (cont.)
9
( )tE
x
y
a
( )tφ
( ) ( )d tt t
dtφ
φ ω ω= ⇒ =
IEEE convention
10
Circular Polarization (cont.)
( )( )
cos
cosx
y
a t
b t
ω
ω β
=
= +
E
E
RHCP
LHCP
( )tE
x
y
a
( )tφ/ 2β π=
/ 2β π= −
/ 2β π= ±
Note: The mechanical angular
velocity is the same as the electrical radian frequency ω.
There is opposite rotation in space and time.
11
Examine how the field varies in both space and time:
Rotation in space vs. rotation in time
( )ˆ ˆ( ) j jkzE z x a y be eβ −= +
( ) ( )ˆ ˆ( , ) cos cosz t x a t kz y b t kzω ω β= − + − +E
Phasor domain
Time domain
Circular Polarization (cont.)
Notice that the rotation in space matches the left hand!
12
A snapshot of the electric field vector, showing the vector at different points.
Circular Polarization (cont.)
RHCP
z
13
Animation of LHCP wave
http://en.wikipedia.org/wiki/Circular_polarization
(Use pptx version in full-screen mode to see motion.)
Circular Polarization (cont.)
Circular polarization is often used in wireless communications to avoid problems with signal loss due to polarization mismatch.
Misalignment of transmit and receive antennas Reflections off of buildings Propagation through the ionosphere
Receive antenna
The receive antenna will always receive a signal, no matter how it is rotated about the z axis.
However, for the same incident power density, an optimum linearly-polarized wave will give the maximum output signal from this linearly-polarized antenna (3 dB higher than from an incident CP wave). The linear antenna “throws away” half of the incident signal.
14
Circular Polarization (cont.)
z
Two ways in which circular polarization can be obtained:
This antenna will radiate a RHCP signal in the positive z direction, and LHCP in the negative z direction.
Method 1) Use two identical antennas rotated by 90o, and fed 90o out of phase.
15
Circular Polarization (cont.)
Antenna 1
1xV =1 90oyV j= − = ∠ −
Antenna 2
+
-
+
-
/ 2β π= −
x
y
Realization of method 1 using a 90o delay line
16
Circular Polarization (cont.)
2 1 / 2l lβ β π− =
ANT01 inZ Z=
Antenna 1
Antenna 2
Feed line
Signal
02 01 / 2Z Z=Power splitter :
02Z
x
y
01Z
01Z 1l
2l
17
An array of CP antennas
Circular Polarization (cont.)
18
The two antennas can realized by using two different modes of a single microstrip or dielectric resonator antenna.
Circular Polarization (cont.)
Method 2) Use an antenna that inherently radiates circular polarization.
Helical antenna for WLAN communication at 2.4 GHz
http://en.wikipedia.org/wiki/Helical_antenna
19
Circular Polarization (cont.)
20
Helical antennas on a GPS satellite
Circular Polarization (cont.)
21
Other Helical antennas
Circular Polarization (cont.)
An antenna that radiates circular polarization will also receive circular polarization of the same handedness, and be blind to the opposite
handedness. (The proof is omitted.)
22
Circular Polarization (cont.)
Note: It does not matter how the receive antenna is rotated
about the z axis.
RHCP wave
RHCP Antenna
2 1 / 2l lβ β π− =
Antenna 1
Antenna 2
Feed line
Output signal
02 01 / 2Z Z=Power combiner :
02Z
x
y
01Z
01Z 1l
2l
z
The two signals add in phase.
23
Summary of possible scenarios
1) Transmit antenna is LP, receive antenna is LP
Simple, works good if both antennas are aligned. The received signal is less if there is a misalignment.
2) Transmit antenna is CP/LP, receive antenna is LP/CP
Signal can be received no matter what the alignment is. The received signal is 3 dB less then for two aligned LP antennas.
3) Transmit antenna is CP, receive antenna is CP
Signal can be received no matter what the alignment is. There is never a loss of signal, no matter what the alignment is. The system is now more complicated.
Circular Polarization (cont.)
Includes all other cases that are not linear or circular
Elliptic Polarization
The tip of the electric field vector stays on an ellipse.
24
( )tE
x
y
( )tφ
(This is proved in Appendix A.)
00
β ππ β
< <− < <
LHEPRHEP
Elliptic Polarization (cont.)
Rotation Property
25
LHEP
RHEP
( )tE
x
y
( )tφ
(This is proved in Appendix B.)
( )( )
cos
cosx
y
a t
b t
ω
ω β
=
= +
E
E
Rotation Rule
Here we give a simple graphical method for determining the type of polarization (left-handed or right handed).
26
First, we review the concept of leading and lagging sinusoidal waves.
27
Two phasors: A and B
0 β π< <
0π β− < <
Note: We can always assume that the phasor A is on the real axis (zero degrees phase)
without loss of generality, since it is only the phase difference between the two phasors
that is important.
Rotation Rule (cont.)
Re
Im
β A
BB Aleads
Re
Im
A
B
B Alags
β
0j
j
A aeB be β
=
=
( ) 0a β π< < LHEP
Observation:
The electric field vector rotates from the leading axis to the lagging axis.
28
Now consider the case of a plane wave.
Rotation Rule (cont.)
Re
Im
βxE
yEy xE Eleads
LHEP
( )tE
x
y
( )tφ
( ) 0b π β− < < RHEP
29
Rotation Rule (cont.)
Observation:
The electric field vector rotates from the leading axis to the lagging axis.
RHEP
( )tE
x
y
( )tφ
Re
Im
xE
yE
y xE Elags
β
Rotation Rule:
The electric field vector rotates from the leading axis to the lagging axis.
30
The rule works in both cases, so we can call it a general rule:
Rotation Rule (cont.)
[ ]ˆˆ (1 ) (2 ) jkyE z j x j e= + + −
What is this wave’s polarization?
Example
31
Rotation Rule (cont.)
x
y
z
xE
zE
Example (cont.)
Therefore, in time the wave rotates from the z axis to the x axis.
32
Rotation Rule (cont.)
[ ]ˆˆ (1 ) (2 ) jkyE z j x j e= + + −
Re
Im
xE
zE z xE Eleads
LHEP or LHCP
Note: and 2x zE E πβ≠ ≠ ± (so this is not LHCP)
33
Rotation Rule (cont.)
Example (cont.)
[ ]ˆˆ (1 ) (2 ) jkyE z j x j e= + + −
x
y
z
xE
zERotation
LHEP
34
Rotation Rule (cont.)
Example (cont.)
[ ]ˆˆ (1 ) (2 ) jkyE z j x j e= + + −
x
y
z
xE
zERotation
AR 1ABCD
= = >major axisminor axis
Axial Ratio (AR) and Tilt Angle (τ)
35
Note: In dB we have ( )dB 10AR 20log AR=
( )tE
τ x
y
A
B
C
D
τ = tilt angle
o o
LHEP
RHEP
45 45
AR cot
0 :0 :
sin 2 sin 2 sinξ
ξ
ξξ
ξ γ β
− ≤ ≤ +
=
><
=where
Axial Ratio and Handedness
tan 2 tan 2 cosτ γ β=
Tilt Angle
Note: The tilt angle τ is ambiguous by the
addition of ± 90o.
1
o
tan
0 90
ba
γ
γ
− ≡
≤ ≤
Axial Ratio (AR) and Tilt Angle (τ ) Formulas
36
tan 2 tan 2 cosτ γ β=Tilt Angle:
The title angle is zero or 90o if:
/ 2β π= ±
Note on Tilt Angle
37
( )tE
x
y
Example
38
Relabel the coordinate system: x xz yy z
→→→ −
Find the axial ratio and tilt angle.
[ ]ˆˆ (1 ) (2 ) jkyE z j x j e= + + −
LHEP
x
y
z
xE
zE
Example (cont.)
39
1ˆ ˆ(1)2
jkzjE x y ej
− += + −
Normalize:
( )1.249ˆ ˆ(1) 0.6324 j jkzE x y e e− = +
or
ˆ ˆ(1 ) (2 ) jkzE y j x j e− = + + −
LHEP
x
z
y
xE
zE
40
71.565oy xE Eβ = ∠ − ∠ =
( ) [ ]1 1tan tan 0.632 0.564 radba
γ − − = = =
Hence
( )1.249ˆ ˆ(1) 0.6324 j jkzE x y e e− = +
o
10.63241.249[rad] 71.565
abβ
==
= =
Example (cont.)
LHEP
x
z
y
xE
zE
41
o16.845τ =
o29.499ξ =
LHEP
Results:
AR 1.768=
o71.565β =
[ ]0.564 radγ =
Example (cont.)
o o
LHEP
RHEP
45 45
AR cot
0 :0 :
sin 2 sin 2 sinξ
ξ
ξξ
ξ γ β
− ≤ ≤ +
=
><
=where
42
o16.845τ =
AR 1.768=
o
o o
16.84516.845 90
τ
τ
=
= +
Note: We are not sure which choice is correct:
We can make a quick time-domain sketch to be sure.
Example (cont.)
( )tE
x
y
τ
LHEP
43
( )( )
cos
cosx
y
a t
b t
ω
ω β
=
= +
E
E
Example (cont.)
o
10.63241.249[rad] 71.565
abβ
==
= =
o
AR 1.76816.845τ
=
=
Given:
Results:
Summary
( )tE
x
y
τ
LHEP
cos( )
cos cos sin siny b t
b t b tω β
ω β ω β
= +
= −
E
so 2
cos sin 1x xy b b
a aβ β
= − −
E EE
22 2cos siny x x
b bba a
β β − = − − E E E
2 22 2 2 2 2cos 2 cos siny x x y x
b b bba a a
β β β + − = −
E E E E E
cosx a tω=E
or
Squaring both sides, we have
Here we give a proof that the tip of the electric field vector must stay on an ellipse.
44
Appendix A
This is in the form of a quadratic expression:
( )2
2 2 2 2 2 2cos sin 2 cos sinx y x yb b ba a
β β β β + + − =
E E E E
22 2 2 22 cos sinx x y y
b b ba a
β β + − + = E E E E
2 2x x y yA B C D+ + =E E E E
or
Collecting terms, we have
45
2 22 2 2 2 2cos 2 cos siny x x y x
b b bba a a
β β β + − = −
E E E E E
Appendix A (cont.)
2
2 22
22
4
4 cos 4
4 cos 1
B AC
b ba a
ba
β
β
∆ = −
= −
= −
224 sin 0b
aβ ∆ = − <
Hence, this is an ellipse.
so
(determines the type of curve)
46
Discriminant:
Appendix A (cont.)
coscos( )
cos cos sin sin
x
y
a tb tb t b t
ωω β
ω β ω β
== +
= −
EE
Here we give a proof of the rotation property.
47
00
β ππ β
< <− < <
LHEPRHEP
Appendix B
LHEP
RHEP
( )tE
x
y
( )tφ
Take the derivative:
[ ]tan cos tan siny
x
b ta
φ β ω β= = −EE
( )( )2 2sec sec sind b tdt aφφ ω ω β = −
Hence
( )( )2 2sin cos secd b tdt aφ β φ ω ω = −
( )
( )
0 0
0 0
ddtddt
a
b
φβ π
φπ β
< < <
− < < >
LHEP
RHEP (proof complete)
48
Appendix B (cont.)