dipole antennas - nptelnptel.ac.in/courses/108101092/week-2-dipole antenna.pdfthe impedance of the n...
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Prof. Girish KumarElectrical Engineering Department, IIT Bombay
(022) 2576 7436
Dipole Antennas
Infinitesimal Dipole
An infinitesimally small current element is
called the Hertz Dipole (Length L< ฮป/50)
Dipole and its field components
in spherical polar co-ordinate
๐ด = ๐ด๐ง ๐ง =๐
4๐๐ผ๐๐๐
๐โ๐๐๐
๐๐๐๐ค๐ก ๐ง
๐(๐ก) = ๐ผ๐๐๐๐ค๐ก ๐๐ง
where, k =2ฯ
ฮป
Assume an infinitesimal current element of
length dl carrying an alternating current Io. The
instantaneous current is
Uniform Current โMagnetic Vector Potential
E and H Fields from Magnetic Vector Potential
Uniform Current โ E and H Fields
Uniform Current โ 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
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
๐ธ๐ = ๐๐๐๐ผ๐๐
4๐๐sin๐
๐ป๐ = ๐๐๐ผ๐๐
4๐๐sin๐
๐ธ๐
๐ป๐= ๐ = 120๐
๐ ๐ = 80๐2๐
๐
2
๐ธ๐ โ ๐ธ๐ = ๐ป๐ = ๐ป๐ = 0
Note : Infinitesimal antenna is not an efficient radiator
Impedance of
free-space
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
Far Field Region (kr>>1)
Small Dipole โ Radiation Resistance
Small dipole current distribution
Small dipole vector potential
๐ผ๐ ๐ฅโฒ, ๐ฆโฒ, ๐งโฒ = ๐๐ง๐ผ0 1 โ2
๐๐งโฒ , 0 โค ๐งโฒ โค ๐ 2
๐๐ง๐ผ0 1 +2
๐๐งโฒ , โ ๐ 2 โค ๐งโฒ โค 0
๐ด ๐ฅ, ๐ฆ, ๐ง =๐
4๐ ๐๐ง
โ ๐ 2
0
๐ผ0 1 +2
๐๐งโฒ
๐โ๐๐๐
๐ ๐๐งโฒ
+ ๐๐ง
0
๐ 2
๐ผ0 1 โ2
๐๐งโฒ
๐โ๐๐๐
๐ ๐๐งโฒ
๐ธ๐ โ ๐๐๐๐ผ0๐๐
โ๐๐๐
8๐๐sin๐
๐ธ๐ โ ๐ธ๐ = ๐ป๐ = ๐ป๐ = 0
๐ป๐ โ ๐๐๐ผ0๐๐
โ๐๐๐
8๐๐sin๐
๐ ๐ =2๐๐๐๐
|๐ผ0|2 = 20๐2
๐
๐
2
For l = ฮป / 10, Rr = 2 ฮฉ
l = ฮป / 4, Rr = 12.3 ฮฉ
Dipoles also have reactive impedance
Input Impedance of Transmission Line
Case 1: ๐๐ฟ= 0, โ Z๐๐= ๐๐0tan๐ฝ๐
Case 2: ๐๐ฟ= โ, โ Z๐๐ =๐0
๐tan๐ฝ๐
Case 3: ๐๐ฟ= ๐0, โ Z๐๐ = ๐0
Where, ๐ฝ =2๐
๐
๐๐ ๐ <๐
4โ tan๐ฝ๐ = +๐ฃ๐
๐
4< ๐ <
๐
2โ tan๐ฝ๐ = โ๐ฃ๐
๐0 ๐L
๐in
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
Half wavelength Dipole
Directivity of half-wavelength dipoleElectric and magnetic fields of
a half-wavelength dipole
Note: Input impedance for ฮป/2 dipole is 73+j42.5ฮฉ.
To make imaginary part equal to zero, the antenna
length is reduced until the input impedance
becomes real.
๐ + ๐ = 0.48๐, where, d is the diameter of wire and d<๐/10
๐ธ๐ โ ๐๐๐ผ0๐
โ๐๐๐
2๐๐
cos๐2 cos๐
sin๐
๐ป๐ โ ๐๐ผ0๐
โ๐๐๐
2๐๐
cos๐2 cos๐
sin๐
๐ ๐ =2๐๐๐๐
|๐ผ0|2 โ 73
๐ท0 = 4๐๐max
๐๐๐๐โ 1.643
D = 2.1 dB
ฮป/2 Dipole Radiation
Resistance
Design of Dipole Antenna
Real Input impedance is < 68ฮฉ.
Current Distribution of Dipole Antenna for Different Lengths
Radiation Pattern of Dipole Antenna for Different Lengths
Dipole Antenna Radiation Pattern for l = 1.25ฮป
Two Dimensional Three Dimensional
Directivity is maximum for a thin dipole of length l = 1.25ฮป
Dipole Antenna Resistance and Directivity
D0 = 3.25
Rr
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%)
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 ฮป
Radiation Pattern at 1.5, 3.75 and 4.5 GHz
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: ฮตr = 4.4, tanฮด =
0.02, h = 1.6mm
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)
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
๐ 4
๐ ๐
๐ 4
Outer
conductor
of coax
Inner
conductor
of coax
Metal
๐ 4 Coaxial Balun (1:1)- Narrow Bandwidth
Sleeve or bazooka balun
Narrow BW
)๐1(balanced
)๐1(unbalanced
Ferrite core
Metal sleeve
Shorted to coaxโs
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.
Microstrip Balun Dipole Antenna for GSM900
Microstrip Balun Dipole Antenna
L = 127 mm, w = 4 mm
FR4 substrate: ฮตr = 4.4, tanฮด =
0.02, h = 1.6mm
BW for |S11| < 10 dB is from 881 to 967 MHz
(covers GSM900 band of 890 to 960 MHz)
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
๐๐๐ = 22๐๐
๐๐๐ = 4๐๐
2-fold dipole antennas are used in Yagi-Uda Antennas for
TV reception using balanced line of Z0 = 300 ฮฉ
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 ฮฉ)