Dipole Antenna

Dipole Antenna

Antenna Antenna EngineeringEngineering

CHO, Yong HeuiCHO, Yong Heui


EM Wave LabEM Wave Lab2

Half-wave dipole antenna

1. Dipole antenna

Current approximation

4/for

4/sin)(

z

zkIzI m

Balun

Coaxial cable

Dipole antenna



Principle

1. Dipole antenna



Electric field

Integration of Hertzian dipole

zdzIr

jkedE

zrjk

)(sin4

)cos(

coszrr

)(sin2

)(sin4

)(sin4

cos

fr

ejIf

r

jke

zdezIr

jkedEE

jkrm

jkr

zjkjkr

1. Dipole antenna



Far-field vs. Fourier transform

Radiation pattern: Fourier transform of source current with respect to wavenumber

)(2

2sin

2sin

2sin

)()(

0

4/

cos4/

0

cos

4/

4/

cos

4/

4/

cos

fk

I

zdezkIzdezkI

zdezkI

zdezIf

m

zjkm

zjkm

zjkm

zjk

1. Dipole antenna



Directivity

Radiation pattern

)()()( fgF

1. Dipole antenna

sin)( g

2sin

cos2

cos)(

f



Radiation pattern:

Dipole antenna with length L

)(sin)( fF

1. Dipole antenna

)(2

2sin

)()(

2/

2/

cos

2/

2/

cos

fk

I

zdezL

kI

zdezIf

m

L

L

zjkm

L

L

zjk

2sin2

coscos2

cos)(

kLkL

f



Antenna length Mainlobe, sidelobe, backlobe, and null Broadside and end-fire antenna

Radiation pattern

1. Dipole antenna

sin

coscoscos)(

LL

F

L = 0.5, 1, 1.5 L = 2, 2.5, 3



Radiation resistance

Radiated power

1. Dipole antenna

0

322

2

0 0

2222

2

sin)(4

sin)(sin2

1

2

1

1

2

1

dfI

ddrfr

I

dSEP

m

m

Sr



Radiation resistance


1. Dipole antenna

rm

mm

mr

RI

II

dfI

P

2

22

0

322

2

1

44.28

)2(Cin8

sin)(4

73rR



Directivity


1. Dipole antenna

)2(Cin8

822

22

2

max2

mr

mm

IP

IE

rU

64.1D78BW

)2(Cin

44

r

m

P

UD



Inductive loading: input Top loading, L-shaped antenna Pig-tail antenna Narrow bandwidth- 10dB return loss or VSWR 2:1

Characteristics of half-wave dipole

1. Dipole antenna

)2/cot(0in LjZZ

)2/tan(

)2/tan(

0

00in LjZZ

LjZZZZ

L

L



Coaxial line Quarter-wave dipole with ground Image theorem Sleeve monopole

Characteristics

2. Monopole antenna



Monopole vs. dipole

2. Monopole antenna

didi0

di

mono0

mono

mono

di2di

in

di

2monoin

monomono

diindi

in

diin

monoin

monoinmono

in

25.0

5.02

5.05.0

DU

U

U

UD

RI

P

I

PR

ZI

V

I

VZ

rr

rrr

r



Design of monopole antenna

2. Monopole antenna

Courtesy to www.freeuk.com

MonopoleConnector

Transition



Tilted dipole antenna High directivity Easy beam scanning

Characteristics

3. Vee dipole



Transmission line mode Antenna mode

Characteristics

4. Folded dipole

DA

TT

T

Z

VI

Z

VI

LjZZ

5.0

5.0

)2/tan(0

DT

DT

AT ZZ

ZZ

II

VZ

2

4

5.0in



: infinity Real input impedance: resonance

Half-wave folded dipole

4. Folded dipole

][292][724

4in

DZZ

TZ

2

2in

2

12

1

5.0

DDD

FF

DF

IZP

IZP

II



Feeding: maximum current Balun (Balanced to Unbalanced)- Coaxial cable- Sleeve balun (bazooka balun)

Balun

5. Feeding



Coaxial line to folded dipole Current division and voltage doubling Ferrite core: transformer

1:4 balun

5. Feeding



Chu limit

6. Antenna limit

12

BWfrac NGQ

G

Dipole Antenna

Business

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