unit 5 loop antennas
TRANSCRIPT
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Loop Antennas
Slides for Amplitude Patterns and
Applications removed
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Loop Antennas
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Loop Antennas
Rectangular, square, circular, triangular or
elliptical
Electrically small loops (Circumference
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Principle of Operation
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Fields from a loop antenna
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The far electric field has only an E component
The small dipoles #2 and #4 are non-
directional in the y-z plane.
/2 /2
0 0
0
2, sin sin
2 sin sin2
j j
r
r
E E e E e
dwhere d
dE jE
The total field E is
in phase-quadrature
with the field
E0 of the
individual dipole
Fields from a loop antenna
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If d
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But l=d (square loop), dr=2d/and A=d2,
The far-field magnetic component is:
22
120 sinI AE
r
2
sinE I AH
r
Fields from a loop antenna
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Comparison of fields from Small loop and Short Dipole
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Equivalence of Short Magnetic Dipole and Small Loop
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Magnetic dipole moment of the dipole = qml,
where qm is the pole strength (magnetic
charge) at each end.
The magnetic current is:
where,Im=I0 ejt
Equivalence of Short Magnetic Dipole and Small Loop
(1)m md
I qdt
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Integrating (1), we obtain
The magnetic moment of the loop isIA.
Equating magnetic moment of the small loop
to that of the dipole,
/ (2)m m
q I j
0
2
; 2 2
240 ...(3)
mm
m
I l ZIA I l j IA j f IA j IA
j
IAor I l j
2
or 240mI A
I l j
In retarded-form
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Use of loop antenna in Direction
Finding
Uses vertical loop antennas
If the loop antenna is perpendicular/parallel to
the plane of the wave, the received signal is
minimum (zero)/maximum.
Ambigiuty due to presence of multiple maxima
(0 and 180) and two minima (90 and 270)
This ambiguity is eliminated using a verticaldipole (that re-shapes the pattern) in addition
with the loop.
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Cross Loop Direction Finder
Rotating the antenna mounted on a ship or an
airplane is difficult
Hence, two fixed loop antennas, mutually
perpendicular to each other are used.
Signals from these antennas are combined in
the radio-goniometer (used to measure the
angle)
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Cross Loop Direction Finder
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Equivalent Circuit of goniometer and
linking sense antenna
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Reception from North-South/East-West, zero/max. emfin loop A and max./zero emf in loop B.
When search coil Ps is rotated in a direction parallel tocoil B max. emf is induced in it and vice-versa.
When signal is at 45 degrees from both coils, the emfinduced in search coil is zero because of cancellation ofemfs induced from coils A and B.
So, if the direction of incoming wave changes, theangle at which zero emf is obtained in search coilchanges accordingly. Search coil points to direction ofincoming signal when zero emf is detected.
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Folded Dipole Antenna
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Z11: self impedance of dipole#1,
Z12=mutual impedance between dipole#1 and dipole#2
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For dipoles of equal radii, I1=I2.
If dipoles are close enough ie., d
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The self impedance of dipole#1 is nothing but
its radiation resistance. Hence
Z11=73.14 ohms
Hence the input impedance of folded dipole is
approximately 292 ohms
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Sleeve Dipole
Asymmetrically driven antenna
Frequency response superior to that of a /2 or full-wavelength
dipole.
2
The input impedance of asymmetrically driven antenna is
cos
,
as
sas
Z
ZZ
k l
where
l displacement of the feed from the centre
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Sleeve Dipole
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The excitation gap voltage is moved to a distance lbecause
of the outer sleeve. The input current is
( ) ( )
,
. ' '
( ) . ' '
( )
in as as
in
as
as
I I z h I z h
where
I input current at the feed of the sleeve dipole in Fig a
I z h current of asymmetric structure at z h in Fig d
I z h current of asymmetric structure at z h in
tan
.
:
( ) ( ) )1
(
'
)
'
(as as as
in as
in as
The input admit ce is
I z h I z h I z h
Fig d
Y Y
V I z h
Sleeve Dipole
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Applications
Sleeve dipoles are the best
performing antennas ever used in
cellphones
The height of the antenna at 850MHzis 176 mm (almost /2)
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Applications
Widely adopted by LAN access points