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A dipole antenna is the simplest antenna but its radiation characteristics are very good. The main drawback of a dipole antenna is very narrow bandwidth. The analysis of a dipole antenna can be performed with integration of Hertzian dipoles.
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Dipole Antenna
Antenna Antenna EngineeringEngineering
CHO, Yong HeuiCHO, Yong Heui
Antenna Antenna EngineeringEngineering
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
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab3
Principle
1. Dipole antenna
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab4
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
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab5
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
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab6
Directivity
Radiation pattern
)()()( fgF
1. Dipole antenna
sin)( g
2sin
cos2
cos)(
f
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab7
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 Antenna EngineeringEngineering
EM Wave LabEM Wave Lab8
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
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab9
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
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab10
Radiation resistance
Half-wave dipole antenna
1. Dipole antenna
rm
mm
mr
RI
II
dfI
P
2
22
0
322
2
1
44.28
)2(Cin8
sin)(4
73rR
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab11
Directivity
Half-wave dipole antenna
1. Dipole antenna
)2(Cin8
822
22
2
max2
mr
mm
IP
IE
rU
64.1D78BW
)2(Cin
44
r
m
P
UD
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab12
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
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab13
Coaxial line Quarter-wave dipole with ground Image theorem Sleeve monopole
Characteristics
2. Monopole antenna
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab14
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
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab15
Design of monopole antenna
2. Monopole antenna
Courtesy to www.freeuk.com
MonopoleConnector
Transition
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab16
Tilted dipole antenna High directivity Easy beam scanning
Characteristics
3. Vee dipole
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab17
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
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab18
: 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
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab19
Feeding: maximum current Balun (Balanced to Unbalanced)- Coaxial cable- Sleeve balun (bazooka balun)
Balun
5. Feeding
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab20
Coaxial line to folded dipole Current division and voltage doubling Ferrite core: transformer
1:4 balun
5. Feeding
Antenna Antenna EngineeringEngineering
EM Wave LabEM Wave Lab2121
Chu limit
6. Antenna limit
12
BWfrac NGQ
G