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8/14/2019 ECE-D Antenna 2
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Radiation from
Half Wave Dipole
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Half wave dipole
Its the simplest and fundamentalantenna, has length of Halfwavelength, ie /2.
Otherwise called Dipole antennaor /2 antenna.
Two metal rods or wires havephysical length of /4 withopposite polarity will act assimple dipole.
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Radiation Fields
We already developed the expressions forCurrent element or Unit lengthelement
A dipole of length /2 can be said as
continuously distributed current element,in other words, A dipole antenna is sub-divided into multiple current element.
E and H fields of a dipole we can derivefrom current element itself, howeverthese multiple current elements willcreate a phase shift between them
Such mechanism is modeled as:Total field =(Element Field)(space factor)
This spacer factor will give the phasedifference conce t, it takes a len th
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A dipole antenna isplaced at origin of3-d space
At a point p, located a(r,,)
From the origin,consider a currentelement is located
a height of z. The field created by
the currentelement will createa phase shift with
respect to origin byzcos
cos'zrR =
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Over all phase shift created by all currentelement is referred as Spacer factor
So the total field can be expressed as
Total field =(Element Field) (space factor)
=
sin
2coscos
2cos
factorSpacer
klkl
sin
2coscos
2cos
2E
klkl
r
eIj
jkro
sin
2coscos
2cos
2H
klkl
r
eIj
jkro
These equations are approximate
- for Far Field conditions
by single current element
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Power density, Radiationdensity
Powerdensity
Radiation
density
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For dipole: l=/2
sin
cos2
cos
2E
r
eIj
jkro
sin
cos2
cos
2H
r
eIj
jkro
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Radiated power density, Radiationresistance
Total radiated power
435.2)2( inC
Radiationresistance .Appr 75will match the 50
,Line to air medium for this reaso/ 2 dipole antenna
become much famous
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l>/2
While antenna length is greater than /2, theSpacer factor changes and creatingconstruction and destructions
destructionsConstructions
Side lob
For l=1.25
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l>>/2, for example: 2 or 5
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Assignment-1
Should be in IEEE format
Clear Reference
Plagiarism
Intro + Literature 3marks
Road map 2marks Conclusion 4marks
Earlier submission(before Feb-5) 1mark.
Two Quizzes 10 marks. First quiz on 28-01-2010.Propagation mechanism + Radiation Mechanism
Last date: Fab 12, 5.30pm.
mailto:[email protected]:[email protected]