Transcript Plane wave

4th year – Electrical Engineering Department
MAIN
PRINCIPLES
OF
RADIATION
Guillaume VILLEMAUD
Antennas – G. Villemaud
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First considerations
Two important points:
Most of antennas are metallic
Huge majority of antennas are based on resonators
In a metal, by default the free electrons move erratically.
When creating a difference of potential (eg sinusoidal), the
internal field then controls the distribution of charges.
Currents and charges are then created as basic sources of
electromagnetic field.
But according to their distribution and relative phases, the
overall field delivered by a metallic element is the sum of all
contributions of these basic sources.
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Radiation mechanism
Charges transmitted over a straight metal at a constant
speed do not produce radiation.
+++
No radiation
If the charges encountered a discontinuity (OC, bend ...) their
speed changes, then there is radiation.
+++
Radiation
In a resonant structure, charges continuously oscillate,
creating a continuous stream of radiation.
High radiation
+++
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Loaded two-wire line
Reminder on transmission lines:
Zr
x
superposition of an
incident and a reflected
wave
Two-wire line closed on a load
i
x
 Ae
 jβ x
 Be
jβ x
Without loss
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Open-ended two-wire line
Open-ended line:
O.C.
y
Line with an open-circuit
i

y
jβ y
Stationary waves

 jβ y

i e
 2 ji sin  y
r
r
v
i
  r sin  y cos t
 y, t 
Zc
i e
r
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Resonant line
C.O.
Line with an open-circuit
i
Stationary waves
 jβ y


i e
 2 ji sin  y
r
r
v
i
  r sin  y cos t
 y, t 
Zc

i e
x
r
jβ y
In practice, when the wires are relatively close, the currents are out of
phase, the total radiated field is close to zero (thank goodness).
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Bended wires
The classical approximation considers that if the arms of the line are
moved away, the current distribution remains the same.
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Radiating dipole
Then we have
inphase currents
for effective
radiation: the
principle of the
dipole antenna
Problem: in practice, there is
mismatch. Then we seek a resonant
antenna having an input impedance
matched to a progressive wave line.
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Reminder on EM fields
Medium characteristics:
To study phenomena of electromagnetic wave propagation, a
medium will be defined by:
Its complex electrical permittivity
 ' j''
Its complex electrical permeability
 'j''
Its conductivity

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(S/m)
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electrical loss
(F/m)
Radiation sources
Currents and charges present in this medium are called
primary sources:
Surface current density
I p (A/m²)
Volume charge density
Qp
(Cb/m3)
These sources create:
Electric and magnetic fields
Other currents and charges
E
(V/m)
H
(A/m)
Ic and Qc
Induction phenomena
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Maxwell’s Equations
In an isotropic and homogeneous medium, we
obtain these equations :

b   h
h
rot e   
t

divd   q
e
rot h   e  
t
c

d   e
ic   e

div b  0
Sources can be distributed as linear, surfacic or volumic
densities.
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Resolution domain
Two distinct areas solving these equations are
considered: in the presence of charges and currents
or out of any charge or current.
The resolution in the presence of charges and currents
is used to determine the field distribution produced by a
linear, surface or volume charges and currents (which
leads to the radiation pattern of the antenna).
The second type of resolution is required to calculate
the electromagnetic waves propagated in free space (or
in a particular medium).
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Sinusoidal source
Still in the case of homogeneous and isotropic
media, with harmonic source the following
equations are obtained:

divD  Q
rot E   j  H
C
 
divB   0
rot H   E  j E
Then we can solve these equations to determine
the field produced by the charges and currents
present on a conductor.
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Relation to the surface
Interface with a perfect conductor
1, 1, 1
n  E1  0
n  H1  I S
n. E 1  Q S
n.H 1  0
E1 H 1
The electric field is always
perpendicular to the conductor.
The magnetic field is always tangent
to the conductor.
The electric field is proportional to the
charges on the surface.
The magnetic field is proportional to
the surface current.
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EM potentials
To assess the effects of an isotropic source at a
point P of space we can introduce the vector and
scalar potentials:

 
Knowing that div B  0 we can write B ( r , t )    A( r , t )

z
P
q
r
o
Vector A is defined in a gradient
approximate, then there is a function
V satisfying:
y
j



A ( r , t )
E ( r , t )   V ( r , t ) 
t
x
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EM potentials
Expressing Maxwell's equations based on the
potential, we obtain the wave equations:
2
 2V
Q
 V   2  
t 

2

2 A
 A   2   I
t
The resolution (based on the complex Green's
functions) provides for a linear distribution:
e  j r
V
  Ql (r )
.dl Scalar potential
4 0 L
r
 

e  j r
A
  I l (r )
.dl
Vector potential
4 L
r
1
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Elementary source
The Hertzian electric dipole is a linear element,
infinitesimally thin, of length dl (<<l) where we can
consider a uniform distribution of currents (infinite

z
speed).
E(r )
P
q r0
+q
r
r1
i(t)
-q
x
charges Qejt
currents jQ
This is a theoretical tool to predict the behavior of any antenna as
the sum of elementary sources.
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Radiated field calculation
The problem is rotationally symmetrical relative to Oz.
The vector potential has only one component Az:

e  jr
Az 
 I m .dl.
4
r
Then we obtain:
Hr  0

H
Hq  0
1
1
 jr  j
Hj 
 I m .dl. sin q .e   2 
4
 r r 
The magnetic field has just one component:
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Hj
Electric field calculation
Then we can deduce the electric field which is produced :
1
1 
 jr  
Er 
 I m .dl. cosq .e  2 
3
2

r
j

r



E Ej  0
1

1 
 jr  j
Eq 
 I m .dl. sin q .e 
 2
3
4
r
r
j

r


Er and Eq
Electric field with two components:
So we end up finally with three components of the radiated
field.
Depending on the distance from the observation point P with
respect to the source, we will do different approximations to
simplify expressions.
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Approximations depending on r
1
1
 jr  j
Hj 
 I m .dl. sin q .e   2 
4
 r r 
1
1 
 jr  
Er 
 I m .dl. cosq .e  2 
3
2
r
j

r


1

1 
 jr  j
Eq 
 I m .dl. sin q .e 
 2
3
4
r
r
j

r


The terms in 1/r represent the radiated field
(predominant when large r) 1/r2 terms give the induced
fields and terms in 1/r3 the electrostatic field.
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Zones of radiation
Spherical
waves
Emitter
Wave
surfaces
Antenna
Plane
waves Wave
surfaces
Feeding line
Very near zone
(some wl)
Near field zone
(Fresnel)
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Far field zone
(Fraunhoffer)
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Zones of radiation
Quasi-constant
Fluctuating
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Decreasing in 1/r²
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Hertzian dipole’s radiation
Far field approximation :
H j (r , t ) 
j
 I  dl  sin q  e j (t  r )
2l r
j

Eq (r , t ) 
 I  dl  sin q  e j (t  r )
2l r 
i(t)
o
Free space

 120  377
o
H ( r , t )
Eq ( r , t )
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Farfield Propagation
Returning to the harmonic equations in the case of
homogeneous, isotropic media containing no
primary sources, we obtain the following equations:

divD   0
rot E   j  H
 
rot H  j E

div B  0
Remark : In this case, we see that the equations in E and H are
almost symmetrical, the only difference being the absence of
charges and magnetic currents. We can then introduce fictitious
magnetic sources for these symmetrical equations. The solution of
the electrical problem then gives the magnetic problem solution and
vice versa.
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Propagation equations
The propagation equations for the fields E and H (expressed in
complex instantaneous values​​) are written as follows:
2 H
 H   2  0
t
 E
 E   2  0
t
2
If propagation is in the direction Oz, it comes:
2
2
2 E
2 E

H

H
and



0


0
2
2
2
2
z
t
z
t
1
represents the propagation speed of the wave.

Knowing that generally we consider that  r  1 (except for ionised or
magnetic medium) we can write :
The ratio
v
1
1
c
c
v




00r
r n
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Solutions
In a sinusoidal steady state regime, these equations admit solutions of the
form:
e(z, t)  E exp j(t  kz) and h(z, t)  H exp j(t  kz)
 2

   (wavenumber)
v
l
The ratio between absolute values of E and H represents the wave
impedance of the considered medium (in ohms):
with :
k




H
E
it’s a real value.
In the air: 377 ohms
We have a fundamental relation:
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
E Hu

Spherical wave –Plane wave
A point source (Q charge) produce radiation of a
spherical wave.
Indeed, solving the equations of potential in the case of a
point source is symmetrical spherical revolution, and
gives solution for:
E (r ) 
1
4
 Q.e
 jr
 j 1 
 r  r 2 
In Farfield, this leads to:
Eo  jr
E (r ) 
e
r
The wave surface is a sphere centered at the point source
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Plane wave approximation
Solutions of Maxwell's equations are numerous (depending
on the initial conditions).
All can be expressed as the sum of plane waves.


E  E0  cost  z  dz
l
E
H
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Propagation direction
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Carried power
x
E
E
z
H
y
When the far field condition is satisfied, the wavefront can
be assimilated to a plane wavefront. The power carried
by the wave is represented by the Poynting vector:
*
1
P  EH
2
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Plane wave propagation
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Polarization of the wave
We know that far-field E and H are perpendicular to each
other and perpendicular to the direction of propagation.
But depending on the type of source used, the orientation of
these vectors in the plane wave can vary.
Based on the variations in the orientation of the field E over
time, we define the polarization of the wave.
In spherical coordinates, the components of the E field of a
plane wave is described
 by:


EEq uq Ejuj
with
Eq  Asinta
and
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Ej Bsintb
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Linear polarization
First hypothesis: components pulse in phase
ab



E sint Auq Buj 
Several possibilities:
horizontal, vertical or slant
polarization
Ej

E
animation
Eq
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Example with hertzian dipole
i(t)
Linear vertical polarization
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Linear horizontal polarization
i(t)
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Example with 2 inphase dipoles
i(t)
Slant linear polarization
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Circular polarization
Second hypothesis: components vibrate in phase quadrature
and magnitudes are equal
ba

 2

E Asintauq costauj 
Ej

E
Eq
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i(t)
Circular polarization
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Animations
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Illustration of Circular polarization
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Elliptic Polarization
3 modes of polarization
– Linear polarization
• vertical, horizontal, slant (plane H or E)
– Circular polarization
• Left-hand or right-hand
– Elliptic polarization
• General definition
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Fundamental theorems
To study the functioning of antennas, four fundamental
theorems are known:




the Lorentz reciprocity theorem
the theorem of Huygens-Fresnel
the image theory
Babinet's principle
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Lorentz reciprocity
If we consider that two distributions of currents I1 and I2 are
the source of E1 and E2 fields, Maxwell's equations allow to
write:
 
 
E2.I1.dvE1.I2.dv
v
v
radiating systems are reciprocal (note only in
passive antennas).
Pf
Pr
Pr
Pf
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Huyghens-Fresnel’s principle
Principle for calculating the radiation at infinity of
any type of source
Arbitrary surface
sources
equivalent surface
sources (electric
and magnetic)
No field
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Application to radar
Principle for bistatic radar
Plane wave
target
P
Observation
point
The field received in P is the sum of the field that would be
received without the obstacle (known) and diffracted by the
obstacle. It is then possible to calculate the inverse of the
surface formed by sources providing such a field.
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Image theory
At an observation point P, the field created by a source + q
placed above a perfect ground plane of infinite dimensions is
equivalent to the field created by the combination of this
charge with its image by symmetry with a charge -q.
x
+q
P
x
+q
-q
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P
Image of currents
The same principle applies to the current sources.
The image is formed by the symmetry of the current
distribution of opposite sign (phase opposition).
x
P
I
x
P
I
I
This is the basis for many applications in antennas
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Babinet’s principle
Babinet's theorem shows the symmetrical appearance
of Maxwell's equations.
H
E
case 1
case 2
The total field of case 1 will be equal to the
diffracted field in case 2 and vice versa.
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Application to antennas
Any slot in a ground plane of large dimension will have
the same behavior that the equivalent metallic antenna
in free space except that the E and H fields are
reversed.
E
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H
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