1 ene 325 electromagnetic fields and waves lecture 11 uniform plane waves
TRANSCRIPT
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ENE 325ENE 325Electromagnetic Electromagnetic Fields and WavesFields and Waves
Lecture 11Lecture 11 Uniform Plane Waves Uniform Plane Waves
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IntroductionIntroduction
From Maxwell’s equations, if the electric field is changing with time, then the magnetic field varies spatially in a direction normal to its
orientation direction
A uniform plane wave, both electric and magnetic fields lie in the transverse plane, the plane whose normal is the direction of propagation
Both fields are of constant magnitude in the transverse plane, such a wave is sometimes called a transverse electromagnetic (TEM) wave.
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http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=52
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Maxwell’s equationsMaxwell’s equations
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Maxwell’s equations in free Maxwell’s equations in free space space
= 0, = 0, rr = 1, = 1, rr = 1 = 1
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Ampère’s law
Faraday’s law
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General wave equationsGeneral wave equations
Consider medium free of charge where For linear, isotropic, homogeneous,
and time-invariant medium,
(1)
(2)
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General wave equationsGeneral wave equations
Take curl of (2), we yield
From
then
For charge free medium
( )
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A A A
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Helmholtz wave equationHelmholtz wave equation
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t t
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t t
For electric field
For magnetic field
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Time-harmonic wave Time-harmonic wave equationsequations
Transformation from time to frequency domainTransformation from time to frequency domain
ThereforeTherefore
j
t
2 ( ) ����������������������������s sE j j E
2 ( ) 0 ����������������������������s sE j j E
2 2 0 ����������������������������s sE E
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Time-harmonic wave Time-harmonic wave equationsequations
or
where
This term is called propagation constant or we can write
= +j
where = attenuation constant (Np/m) = phase constant (rad/m)
2 2 0 ����������������������������s sH H
( ) j j
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Solutions of Helmholtz Solutions of Helmholtz equationsequations
Assuming the electric field is in x-direction and the wave is propagating in z- direction
The instantaneous form of the solutions
Consider only the forward-propagating wave, we have
Use Maxwell’s equation, we get
0 0cos( ) cos( )
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x xE E e t z a E e t z a
0 cos( )
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xE E e t z a
0 cos( )
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yH H e t z a
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Solutions of Helmholtz equations Solutions of Helmholtz equations in phasor formin phasor form
Showing the forward-propagating fields without time-harmonic terms.
Conversion between instantaneous and phasor form
Instantaneous field = Re(ejtphasor field)
0
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z j zs xE E e e a
0
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z j zs yH H e e a
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Intrinsic impedance Intrinsic impedance
For any medium,
For free space
x
y
E jH j
0 0
0 0
120 x
y
E EH H
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Propagating fields Propagating fields relationrelation
1
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s s
H a E
E a H
where represents a direction of propagation.a
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Propagation in lossless-charge Propagation in lossless-charge free mediafree media
Attenuation constant = 0, conductivity = 0
Propagation constant
Propagation velocity
for free space up = 3108 m/s (speed of light)
for non-magnetic lossless dielectric (r = 1),
1
pu
p
r
cu
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Propagation in lossless-Propagation in lossless-charge free mediacharge free media intrinsic impedance
Wavelength
2
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Ex1Ex1 A 9.375 GHz uniform plane wave is A 9.375 GHz uniform plane wave is propagating in polyethelene (propagating in polyethelene (rr = = 2.26). 2.26). If the amplitude of the electric field If the amplitude of the electric field intensity is 500 V/m and the material is intensity is 500 V/m and the material is assumed to be lossless, findassumed to be lossless, finda) phase constant
b) wavelength in the polyethelene
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c) propagation velocity
d) intrinsic impedance
e) amplitude of the magnetic field intensity
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Propagation in Propagation in dielectricsdielectrics
Cause finite conductivity polarization loss ( = ’-j” )
Assume homogeneous and isotropic medium
' "( ) ������������������������������������������H E j j E
" '[( ) ] ����������������������������H j E
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Propagation in Propagation in dielectricsdielectrics
",eff Define
from2 ( ) j j
and 2 2( ) j
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Propagation in Propagation in dielectricsdielectrics
We can derive2
( 1 1)2
2
( 1 1)2
and 1
.1 ( )
j
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Loss tangentLoss tangent
A standard measure of lossiness, used to classify a material as a good dielectric or a good conductor
"
' 'tan
eff
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Low loss material or a good Low loss material or a good dielectric (tandielectric (tan «« 1) 1)
If , consider the material
‘low loss’ , then
1
2
(1 ).2
jand
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Low loss material or a good Low loss material or a good dielectric (tandielectric (tan «« 1) 1)
propagation velocity
wavelength
1
pu
2 1
f
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High loss material High loss material or a good or a good conductor conductor (tan(tan »» 1) 1)
In this case , we can
approximate
1
2 f
45 .
jje
therefore
2
1 1)
and
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High loss material High loss material or a or a good conductor good conductor (tan(tan »» 1) 1)
depth of penetration or skin depth, is a distance
where the field decreases to e-1 or 0.368 times of
the initial field
propagation velocity
wavelength
1 1 1m
f
pu
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Ex2Ex2 Given a nonmagnetic material Given a nonmagnetic material having having rr = 3.2 and = 3.2 and = 1.5= 1.51010-4-4
S/m, at S/m, at ff = 3 MHz, find = 3 MHz, find a) loss tangent
b) attenuation constant
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c) phase constant
d) intrinsic impedance
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Ex3Ex3 Calculate the followings for Calculate the followings for the wave with the frequency the wave with the frequency ff = 60 = 60 Hz propagating in a copper with Hz propagating in a copper with the conductivity, the conductivity, = 5.8 = 5.8101077 S/m: S/m:
a) wavelength
b) propagation velocity
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c) compare these answers with the same wave propagating in a free space