Lecture VII Antennas & Propagation -1- Antennas & Propagation Mischa Dohler King’s College London Centre for Telecommunications Research.

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Antennas

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Propagation

Mischa DohlerMischa Dohler

King’s College LondonKing’s College London

Centre for Telecommunications ResearchCentre for Telecommunications Research

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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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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

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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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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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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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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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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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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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Slot on Waveguide Walls

TE10 mode

Radiation is maximum at maximal interrupted current

Radiation

No Radiation

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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 Shapes

Rectangular Dipole

EllipticalCircular Ring

Triangular Analysing Methods

-Transmission Line

- Cavity

- Maxwell Equations

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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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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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Applications

1. 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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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 -