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Prof. Girish KumarElectrical Engineering Department, IIT Bombay
(022) 2576 7436
Dipole Antennas
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Infinitesimal Dipole
An infinitesimally small current element is
called the Hertz Dipole (Length L< Ξ»/50)
Dipole and its field components
in spherical polar co-ordinate
π΄ = π΄π§ π§ =π
4ππΌπππ
πβπππ
ππππ€π‘ π§
π(π‘) = πΌππππ€π‘ ππ§
where, k =2Ο
Ξ»
Assume an infinitesimal current element of
length dl carrying an alternating current Io. The
instantaneous current is
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Uniform Current βMagnetic Vector Potential
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E and H Fields from Magnetic Vector Potential
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Uniform Current β E and H Fields
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Uniform Current β Near and Far Fields
Near Field Region
Far Field Region
Near Reactive Field Region
Near Radiative Field Region
r > where d is the maximum dimension
of the antenna
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Far Field Region (kr>>1)
Uniform Current - Radiation Pattern
max
0
34
2rad
UD
P
Directivity
E-plane radiation pattern
H-plane radiation pattern
3-D radiation pattern
πΈπ = ππππΌππ
4ππsinπ
π»π = πππΌππ
4ππsinπ
πΈπ
π»π= π = 120π
π π = 80π2π
π
2
πΈπ β πΈπ = π»π = π»π = 0
Note : Infinitesimal antenna is not an efficient radiator
Impedance of
free-space
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Small Dipole Antenna
A current element whose length is /50 < l /10 is called small dipole antenna
A small dipole Antenna Approximate Triangular Current distribution
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Far Field Region (kr>>1)
Small Dipole β Radiation Resistance
Small dipole current distribution
Small dipole vector potential
πΌπ π₯β², π¦β², π§β² = ππ§πΌ0 1 β2
ππ§β² , 0 β€ π§β² β€ π 2
ππ§πΌ0 1 +2
ππ§β² , β π 2 β€ π§β² β€ 0
π΄ π₯, π¦, π§ =π
4π ππ§
β π 2
0
πΌ0 1 +2
ππ§β²
πβπππ
π ππ§β²
+ ππ§
0
π 2
πΌ0 1 β2
ππ§β²
πβπππ
π ππ§β²
πΈπ β ππππΌ0ππ
βπππ
8ππsinπ
πΈπ β πΈπ = π»π = π»π = 0
π»π β πππΌ0ππ
βπππ
8ππsinπ
π π =2ππππ
|πΌ0|2 = 20π2
π
π
2
For l = Ξ» / 10, Rr = 2 Ξ©
l = Ξ» / 4, Rr = 12.3 Ξ©
Dipoles also have reactive impedance
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Input Impedance of Transmission Line
Case 1: ππΏ= 0, β Zππ= ππ0tanπ½π
Case 2: ππΏ= β, β Zππ =π0
πtanπ½π
Case 3: ππΏ= π0, β Zππ = π0
Where, π½ =2π
π
ππ π <π
4β tanπ½π = +π£π
π
4< π <
π
2β tanπ½π = βπ£π
π0 πL
πin
l
For Short-circuit, ZL = 0, Zin is inductive, so T-Line represents inductance
Open-circuit, ZL = , Zin is capacitive, so T-Line represents capacitance
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Half wavelength Dipole
Directivity of half-wavelength dipoleElectric and magnetic fields of
a half-wavelength dipole
Note: Input impedance for Ξ»/2 dipole is 73+j42.5Ξ©.
To make imaginary part equal to zero, the antenna
length is reduced until the input impedance
becomes real.
π + π = 0.48π, where, d is the diameter of wire and d<π/10
πΈπ β πππΌ0π
βπππ
2ππ
cosπ2 cosπ
sinπ
π»π β ππΌ0π
βπππ
2ππ
cosπ2 cosπ
sinπ
π π =2ππππ
|πΌ0|2 β 73
π·0 = 4ππmax
ππππβ 1.643
D = 2.1 dB
Ξ»/2 Dipole Radiation
Resistance
Design of Dipole Antenna
Real Input impedance is < 68Ξ©.
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Current Distribution of Dipole Antenna for Different Lengths
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Radiation Pattern of Dipole Antenna for Different Lengths
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Dipole Antenna Radiation Pattern for l = 1.25Ξ»
Two Dimensional Three Dimensional
Directivity is maximum for a thin dipole of length l = 1.25Ξ»
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Dipole Antenna Resistance and Directivity
D0 = 3.25
Rr
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Flat Dipole Antenna
Length of each segment = 50 mm
Width = 4mm, Gap = 2mm
BW for |S11| < 10 dB is
from 1.39 to 1.54 GHz
(150 MHz, 10.2%)
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Flat Dipole Antenna Pattern and Directivity
Directivity of 4.8 dB is
maximum at 3.75 GHz
where length of dipole
is approx. 1.25 Ξ»
Radiation Pattern at 1.5, 3.75 and 4.5 GHz
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Printed Dipole Antenna
BW = 1.14 to 1.28 GHz (140 MHz, 11.6%)
Length of each segment = 50 mm
Width = 4mm, Gap = 2mm
FR4 substrate: Ξ΅r = 4.4, tanΞ΄ =
0.02, h = 1.6mm
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Broadband Dipole Antenna
Bandwidth of dipole antenna is directly proportional to its diameter
Cylindrical dipole antenna
(can use hollow pipe also) Biconical dipole antenna
(can use wire grid also)
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Balun Design
Devices that can be used to balance inherently unbalanced systems by cancelling
or choking the outside current, are known as baluns (balance to unbalance).
Coaxial line
Shorted
together
π 4
π π
π 4
Outer
conductor
of coax
Inner
conductor
of coax
Metal
π 4 Coaxial Balun (1:1)- Narrow Bandwidth
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Sleeve or bazooka balun
Narrow BW
)π1(balanced
)π1(unbalanced
Ferrite core
Metal sleeve
Shorted to coaxβs
outer conductorCoaxial line
Ferrite core balun
Wide BW
Balun Design (Contd.)
Ferrite core maintains high impedance levels over a wide frequency range.
A good design can provide bandwidths of 10 to 1 whereas coil coaxial
baluns can provide bandwidths of 2 or 3 to 1.
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Microstrip Balun Dipole Antenna for GSM900
Microstrip Balun Dipole Antenna
L = 127 mm, w = 4 mm
FR4 substrate: Ξ΅r = 4.4, tanΞ΄ =
0.02, h = 1.6mm
BW for |S11| < 10 dB is from 881 to 967 MHz
(covers GSM900 band of 890 to 960 MHz)
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Folded Dipole Antenna
The impedance of the N fold folded dipole is
N2 times greater than that of an isolated
dipole of the same length as one of its side.
Impedance for 2-fold dipole antenna is
πππ = 22ππ
πππ = 4ππ
2-fold dipole antennas are used in Yagi-Uda Antennas for
TV reception using balanced line of Z0 = 300 Ξ©
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Dipole Antenna Applications
Compact Dipole Antenna for RFID
Chip
Folded Broadband Dipole Antenna for
RF Harvesting
(Triangular shape for broadband and
multi-fold gave Zin = 750 Ξ©)