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FOLDED DIPOLE ANTENNA

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Folded dipole antenna is made with twoor more parallel and closely spaceddipole antennas connected together attheir ends with one of dipole antennacentre fed.

The basic dipole can be /\/2 long butfolded structures shows that it forms a

complex circuit. It is the heart of many advanced antenna

designs because it is mechanically morerugged than a simple dipole and

bandwidth is about 10% greater then the

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The size of the dipole antenna is morebut gain and directivity is less, so if the

dipole antenna is folded and length afterfolding is /\/2,than its size is reducedand radiation performance improves.

As the no. of dipole elements increase

the impedance radiation efficiencyincreases. In the folded dipole antenna,when one element is center fed thenother elements are inductively coupled

and transfer the radiation from one dipoleto adjacent dipole and amplify theradiation. As the no. of dipoles increases,the gain & directivity increases.

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Z=73

SIMPLE DIPOLE

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Z=273

FOLDED DIPOLE WITH TWO

ELEMENTS

FOLDED DIPOLE WITH THREE

ELEMENTS

Z=373

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RANGE-HF,VHF,UHF,Range.

Structure

1.it is a single antenna but consist ofmore elements .

2.The spacing between the arms is very

small and is of order of /\/100.3.When the power is fed through first

then second is inductively coupled at

ends.

DESIGN-

If the folded dipole consist of n parallel

and closely spaced dipoles the

impedance -

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V/2=IZ+IZ

WHEN ICurrent in dipole 1

ICurrent in dipole 2ZSelf impedance of dipole

1.

ZMutual impedance

between 1&2.if I= I=I for equal dimensions

V/2=I(Z+Z)

Since two dipoles are very small and spacingis very small Z+Z

V/2=2IZ

Z=V/I=4Z =473 =273

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In the same way for three arm antenna

Z=973=373

So for n arms Z=n73

Bandwidth of folded dipole is 10% which i

greater than simple dipole.

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Advantages

High input impedance Greater bandwidth

Low cost

Television reciever,short distance

communication, radar etc.

Applications

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