Post on 17-Jan-2016
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-1-
Antennas
&
Propagation
Wu Qun
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-2-
Overview of Lecture VII
- Review of Lecture VIReview of Lecture VI
- Frequency Independent AntennasFrequency Independent Antennas
- Basics of Aperture AntennasBasics of Aperture Antennas
- Horn AntennaHorn Antenna
- Slot AntennaSlot Antenna
- Microstrip (Patch) AntennaMicrostrip (Patch) Antenna
- Parabolic AntennaParabolic Antenna
- Antennas: Practical ConsiderationsAntennas: Practical Considerations
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-3-
Review
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-4-
Wire Antennas
1. Hertzian Dipole
2. Finite Length Dipole
3. Antenna Array
4. Uda-Yagi
5. Turnstile
6. Loop
7. Helix
8. Quadrifilar Helix
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-5-
VHF TV Receive Antenna
Uda-Yagi Antenna
5-6 Directors
Folded Dipole
Driver
Sheet Reflector
Feeding Mast
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-6-
Helical Antenna
x
z
y
Circumference C
Axial Mode Radiation (endfire) appears if:
3/4 < C/ < 4/3
1. Narrow Mainbeam with minor
sidelobes
2. HPBW 1/(Number of turns)
3. Circular Polarisation
(orientation helix
orientation)
4. Wide Bandwidth
5. No coupling between elements
6. Supergain Endfire Array
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-7- Frequency Independent
Antennas
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-8-
Rumsey’s Principle
All antenna characteristics so far were always scaled with respect
to . Thus, changing changes the characteristic.
The impedance and pattern properties
of an antenna will be frequency
independent if the antenna shape is
specified only in terms of angles and
the antenna itself is infinite.
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-9-
Rumsey’s Principle
Scaling through angles self-scaling
Infinite size problem of realisation
Finite Bowtie Antenna
Cur
rent
sho
uld
deca
y fa
st
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-10-
Log-periodic toothed Antenna
Effectively infinite current decays fast
Current decays fast introduce discontinuities
Discontinuities destroy self-scaling nature
Self-scaling nature log-periodic toothed antenna
Log-periodic sheet Log-periodic wire
Characteristic will be repeated at (discrete) nf1.
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-11-
Log-periodic Dipole Array
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-12-
Spiral Antenna
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-13-
Fractal Antenna
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-14-
Aperture Antennas
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-15-
Huygen’s Principle
Any wavefront can be considered to be the
source of secondary waves that add to produce
distant wavefronts.
x
y
z
P
J,
en
r’
r
Surface
jekj
d
4 r'r'snr'sn
kr'
eeHeeEer'
E
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-16-
Aperture Plane
Towards infinity Aperture Plane
Closing Hemisphere
- E-field vanishes on the
Hemisphere at infinity.
- Total field is derived from
the knowledge of the field
on the aperture plane.
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-17-
Rectangular Aperture
φθ ee
E
cos1coscos1sin
sinsin21
sinsin21
sin
cossin21
cossin21
sin
4 0
kb
kb
ka
kabaE
r
ekj
jkr
x
z
Pr’
r
y
b/2
a-a/2
yeE 0EA
xeH /0EA
Polarisation in the far field is the
same as in the aperture.
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-18-
Parameter Rectangular Aperture
cos1
sin21
sin21
sin
4 0
kb
kbbaE
r
ekjE
jkr
yz
y-z plane:
cos1
sin21
sin21
sin
4 0
ka
kabaE
r
ekjE
jkr
yz
x-z plane:
byz
886.0HPBW
axz
886.0HPBW
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-19-
Circular Aperture
φθ ee
E
cos1coscos1sin
sin
sin
212
0 ka
kaJaE
r
ekj
jkr
x
z
Pr’
r
y
a
yeE 0EA
xeH /0EA
Polarisation in the far field is the
same as in the aperture.
J1(x) is the first order Bessel Function of first kind.
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-20-
Parameter Circular Aperture
cos1
sin
sin
212
0
ka
kaJaE
r
ekjE
jkr
yz
y-z plane:
x-z plane:
a2
58HPBW
cos1
sin
sin
212
0
ka
kaJaE
r
ekjE
jkr
xz
Large Apertures:
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-21-
Directivity
baDrec 2
4
Rectangular Aperture:
22
4aDcirc
Circular Aperture: Real Physical AreaeAD
2
4
Definition
Thus, for the uniform rectangular and circular aperture the
physical area is equal to the effective area.
phape AA Non-uniform apertures or fields:
ap … Aperture Efficiency
Aperture Antennas: 30-90%
Horn Antennas: 50%
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-22-
Horn Antennas
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-23-
Horn Antennas
E-Plane
sectoral horn
H-Plane
sectoral horn
Pyramidal
horn
Excitation: TE10 mode
TE10
Impedance Matching
through flare
Gradual Transmission with
minimised reflection
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-24-
Specifications
1. Directive Radiator
2. Primary feed for parabolic reflectors
3. High gain, wide bandwidth and simple
4. Particularly used in microwave region (>1GHz)
5. Fan radiation patterns
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-25-
Slot Antennas
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-26-
Slot Antennas
-x
z
y
w
xeE
zLk
w
VA 2
1sin
L
φeE
sin21
coscos21
cos)(
kLkL
r
eVjr
jkr
22
354764
metalair ZZBookers Principle:
5.4273 jZdipole 211363)2/( jZslot
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-27-
Slot on Waveguide Walls
TE10 mode
Radiation is maximum at maximal interrupted current
Radiation
No Radiation
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-28-
Applications
1. Slot Antennas are used in fast-moving vehicles.
2. The slot-length is usually /2
3. Particularly used in microwave region (>1GHz)
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-29- Microstrip (Patch)
Antennas
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-30-
Patch Structure
Substrate
PatchFeed
r
L
t
d
- - - -
- - - -
+ + + +
+ + + +
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-31-
Patch ShapesRectangular Dipole
EllipticalCircular Ring
Triangular Analysing Methods
-Transmission Line
- Cavity
- Maxwell Equations
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-32-
Application & Performance
1. It is applied where small antennas are required:
aircrafts, mobiles, etc
2. Due to shape variations they are versatile in
polarisation, pattern, impedance, etc.
3. They have a low efficiency, spurious feed
radiation and a narrow bandwidth
4. They usually operate in broadside regime
5. /3 < L < /2 and 2 < r < 12
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-33- Parabolic Reflector
Antennas
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-34-
1. Uda-Yagi: 15dB
2. Helical Antenna: 15dB
3. Antenna Arrays high gains many
elements
4. Horn: high gains large size
Large Gains
Complicated Feeding
Artificially increase size
- (re-) transmitted waves are in phase
- (re-) transmitted waves are as parallel as possible
Aperture increasing Reflector
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-35-
Parabolic Reflector
Feed
Parabolic DishParallel and in-phase waves
eAD 2
4
phape AA Non-uniform fields due to aperture blocking etc
ap … Aperture Efficiency = 80%
22
4rD
r
- Dish has to be 100% parabolic
- Feeder shouldn’t block too much
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-36-
Applications1. Used where high gains are required:
Cosmic Radiation, etc.
2. Navigation
1. Beam is slightly steerable
2. Deviation from perfect surface can be made
<1mm
3. Diameters are usually 100m-300m
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-37-
Practical Considerations
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-38-
Practical Considerations
- The Quality Factor Q
- Electrically Small Antennas
- Physically Small Antennas
- Imperfect Ground
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-39-
Feeding
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-40-
‘Exotic’ Antennas
- Fractal Antennas
- Light Antennas
- Gravity Antennas
Everything what propagates can be transmitted.
Everything what can be transmitted can be received.
- EM waves, sound, smell, light, gravity and maybe 6th sense -