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-1-

Antennas

&

Propagation

Mischa DohlerMischa Dohler

King’s College LondonKing’s College London

Centre for Telecommunications ResearchCentre for Telecommunications Research

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-2-

Overview of Lecture VI

- Review of Lecture VReview of Lecture V

- Antenna Analysis and SynthesisAntenna Analysis and Synthesis

- Uda-Yagi AntennaUda-Yagi Antenna

- Turnstile AntennaTurnstile Antenna

- Loop AntennaLoop Antenna

- Helical AntennaHelical Antenna

- Quadrifilar Helix AntennaQuadrifilar Helix Antenna

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-3- Review

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-4-

Mutual Impedance

2L

221,21

21 d)0()0(

1zzIzE

IIZ z

Approximated current distribution:

zLkIzI 2max,22 2

1sin)(

Electromagnetic Field in the Near Field:

r

eTk

R

e

R

eIjPE

jkrjkRjkR

z cos23021

max21,

21

122121 XjRZ

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-5-

Linear Antenna Array

x

y

z

P(r, , )

cossincos1

cos2

mixed elevation & azimuth

pure elevation

d

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-6-

Principle of Pattern Multiplication

Individual Pattern (of 1 antenna element)

120sin2

1 fPr

ejH

jkr

HE

ARRAY FACTOR (different for each Array)

1

1

01

ndkjN

nn eIf

Constant (similar to all structures)

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-7-

Uniform Array

1

0

N

n

unjeufArray

Factor1 kdu

1

1

01

ndkjN

nn eIf nj

n eI 1

cos kd

Maximum (Main Beam) for u = 0 :

Broadside Array Endfire Array

kd

cos

90 0 0kd

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-8-

Array Polynomial

Nulls on unity circle indicate no

radiation in that particular direction!

)(11

110 zPzAzAAf NN

N

121)( Nzzzzzzzf

u=0

u=/2

1

Walking along the circle is like

walking around the array

Feeding Current:

x

0arg0

AjeA 1arg1

AjeA 2arg2

AjeA 3arg3

AjeA 4arg4

AjeA

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-9- Pattern Synthesis

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-10-

Odd Array

- Odd Array with N = 2m + 1

mm

mm zAzAzAAf 2

210

mmm

mm zAAzAzAf 21

10

Feeding Current:

x

0arg0

AjeA 1arg1

AjeA 2arg2

AjeA 3arg3

AjeA 4arg4

AjeA

N = 2*2 + 1 m=2

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-11-

Fourier Coefficients

mmm

mm zAAzAzAf 21

10

- Symmetric feeding:

0cAm

nnnm jdcA

nnnm jdcA

- Trigonometric Series with &

m

nnn nudnuccf

10 sincos

2

12

cos kdujuez

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-12-

Synthesis Procedure

1. Specify the Array Factor f() either

graphically or analytically

2. Find the Fourier series expansion

coefficients of f()

3. Relate the coefficients to the feeding

current amplitude and phase.

Example, see blackboard.

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-13- Uda-Yagi Antenna

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-14-

3-element Uda-Yagi

x

y

z

Driver

Endfire Regime

reflector 5% longerReflector

Director

d2

d1

director 5% shorter

d=d1=d2: 0.15 - 0.25

Directivity 9dB

highly frequency sensitive

significant backlobe radiation

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-15-

2 - element antenna

1 driven element 1 parasitic element (reflector/director)

1221111 ZIZIV

1212220 ZIZI je

Z

ZII

22

1212

2212

12

1212 arctan R

X

22

2222 arctan R

X

Pattern Multiplication Principle (although not strictly applicable)

1

1

01

ndkjN

nn eIf 1

211 jkdeIIf

1 fPH HE

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-16-

E-field in the Azimuth-plane: coscos1

cos

21 jkdeIICE

cos

22

121 1 kdje

Z

ZICE

Maximum Radiation corresponding tokd

cos

kd

Reflector

180

kd

Director

0

Reflector - Director

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-17-

Maximum Radiation corresponding tokd

cos

kd

Reflector

180

kd

Director

0

Reflector – Director Length

2212,ZZf

insensitive to d/

d1 = d2 = d

sensitive to L/

Reflector 5% longer

Director 5% shorter

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-18-

Reflector

0.5

1

1.5

2

30

210

60

240

90

270

120

300

150

330

180 0

Reflector Driver

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-19-

Director

0.5

1

1.5

2

30

210

60

240

90

270

120

300

150

330

180 0

DirectorDriver

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-20-

0.56192

1.1238

1.6858

2.2477

2.8096

30

210

60

240

90

270

120

300

150

330

180 0

3 - Element Uda-Yagi

DirectorDriverReflector

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-21-

Application of Uda-Yagi

The Uda-Yagi is the most popular receiving

antenna in VHF-UHF due to:

1. Simple feeding system design

2. Low cost

3. Light weight

4. Relatively high gain

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-22-

Application of Uda-Yagi

Higher frequencies cause higher propagation

losses. Thus higher gains with more directors are

required.

FM-Radio (88MHz-108MHz) 3 element UY

TV (low) (54MHz-88MHz) 3 element UY

TV (high) (174MHz-216MHz) 5-6 element UY

TV (470MHz-890MHz) 10-12 element UY

VHF

UHF

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-23-

Practical Design Criteria

1. Closer spacing between elements results in higher

front-to-back ratio with a broader main beam.

2. Wider spacing yields the opposite.

3. Wider spacing has a greater bandwidth.

4. Uda-Yagi has broader bandwidth if reflector is longer

than optimum and director shorter.

5. Folded dipole as driven element to gain more radiation

power and broader bandwidth.

6. To broaden bandwidth reflector should be replaced by

flat sheet (or wire grid).

7. Tilted fan dipole for broader bandwidth.

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-24-

VHF TV Receive Antenna

Man-made noise was found to be preferably vertical polarised.

TV broadcast is horizontally polarised!

5-6 Directors

Folded Dipole

Driver

Sheet Reflector

Feeding Mast

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-25- Corner Reflector

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-26-

Application of Corner Reflector

Tilted Dipole in the Corner

Reflector produces an

elliptically polarised wave.

Application

- Communication through ionosphere (Faraday Rotation)

- Minimises clutter echoes from raindrops

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-27- Turnstile Antenna

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-28-

Turnstile Antenna

Small Cross-Dipole

with quadrature

current feeding:

x

r

y

z

P

dLA B

^

sin4

θEA

dlIr

ekjr z

jkr

^^

sincoscos4

φθEB

dlIr

ekjr x

jkr

xz jIII 0

rrr BA EEE

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-29-

Polarisations

^^

0 sincoscossin4

φθE

jjdlIr

ekjr

jkr

x-z plane ( = 0°)

^

0 cossin4

θE

jdlIr

ekjr

jkr

Linearly Polarised

x-y plane ( = 90°)

^^

0 sin4

φθE

jdlIr

ekjr

jkr = 0° Linear Polarisation

0° < < 90° Elliptical

= 90° Circular

y-z plane ( = 90°)

^^

0 sin4

φθE jdlIr

ekjr

jkr

= 0° Linear Polarisation

0° < < 90° Elliptical

= 90° Circular

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-30-

Radiation Pattern

3-D Pattern of infinitesimal Turnstile Antenna

x

y

z

2-D x-z plane Field Pattern of Turnstile Antennas

x

z

x

zInfinitesimal

Turnstile

Finite Length

Turnstile

Radiation in all

directions!

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-31-

Application

1. Circular polarisation in Broadside direction:

Satellite Communication

Radar Application

2. Communication of unstabilised space-crafts

due to radiation property in all directions.

3. In x-z plane almost circular radiation pattern:

TV-broadcast transmit antenna

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-32- Loop Antenna

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-33-

Loop Antennas (rectangular, loop)

Circular Loop

x

r

y

z

P

a

10 cos2

nn nBB

VjI

Radius of wire: b

Loop coefficients

B0, Bn see graph.

Small Circular Loop

sin0

2

Ir

earH

jkr

rHrE

The Loop pattern has exactly the

same shape as that of a Hertzian

Dipole, where the electric and

magnetic fields are interchanged.

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-34-

Parameters of the Loop

22

02

224

sin32

Iak

URadiation intensity U

20

224

12I

akP

Radiation Power P

22220 akRr Radiation Resistance Rr

2sin5.1 gDirective Gain g

210

10

222

222

ba

ak

ak

RR

Re

Lr

r

Radiation Efficiency e

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-35-

Application

1. Bad transmitter, but spatially very compact:

Low Frequency AM receiver (HiFi)

Connection to high impedance to give high induced voltage.

Ferrite as kernel will give even better performance.

Multiple loop turns to increase radiation resistance

2. Directional Finder (combined with dipole):

xy

z

x

y

- +

Dipole

Loop

Resultant

Pattern

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-36- Helical Antenna

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-37-

Helical Antenna

x

zThe Helical Antenna was

invented by John Kraus

in 1946. (see his books)

Diameter D

Turn spacing S

Circumference C

Pitch Angle

Operational Modes

Normal Mode Radiation Axial Mode Radiation

Ground Plane > /2

Number of turns N

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-38-

Normal Mode Radiation

x

z

Diameter D

Entire Helix

Length L

y

Normal Mode Radiation (broadside) appears if:

D << entire L <<

Current is sinusoidal

along wire, thus

radiation from a loop

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-39-

Axial Mode Radiation preferred mode

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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-40-

Parameter of Axial Mode Radiation

SNC

365HPBW

HPBW

3

2

5

CSNG

Gain

C

Zin 140Input Impedance

N

N

2

12AR

Axial Ratio (Polarisation)

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-41-

Application

1. High gain, large bandwidth, simplicity,

circular polarisation in AXIAL MODE:

Space Communication (200-300MHz)

2. Arrays of Helixes with higher gain (they hardly couple!)

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-42-

Quadrifilar Helix

Antenna

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-43-

Quadrifilar Helix Antenna

The Helical Antenna was invented by

Kilgus in 1968. (see his papers)

1. Used for communication between

mobile user and non-

geostationary satellite systems

2. Gives Circular Polarisation in all

directions, thus becomes

independent of elevation angle of

satellite.