Unit 5 Loop Antennas

8/2/2019 Unit 5 Loop Antennas

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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



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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



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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

Unit 5 Loop Antennas

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