ANTENNAS AND MICROWAVE
ENGINEERING
EC8701
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Linear Antenna
• Any Linear Antenna may be considered as consisting
of large numbers of very short conductors connected
in series.
• A short conductor is often called a Short Dipole.
• A short dipole is always of finite length.
• If the dipole is vanishingly short, it is called an
infinitesimal dipole.
• In general, L<<λ.
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The Short Dipole
+q
d
L
Transmission
Line
Short Dipole
L
I
-q
Equivalent of Short Dipole
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Short Dipole with Coordinate System
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Geometry of Short Dipole
Z
Dipole
dz
L
z
s1
s
r
θ
P
s2
Y
Y
d
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Retardation Effect
In radiating systems, propagation time is a
matter of great importance.
When current is flowing in the short dipole,
the effect of current is not felt instantaneously
at the point P.
Only after an interval equal to the time
required for the disturbance to propagate over
the distance r.
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Retarded Current
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Vector & Scalar Potentials
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Types of Losses
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Potential due to Dipole
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Short Dipole when r>>L
To a
distant
point P
Z
s1
L/2Cosθ
r
θ
L
s2
Y
d
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Radiation Pattern
Far Field Pattern
Near Field Pattern
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Radiation Pattern
The pattern is independent of ɸ.
The space pattern is Doughnut-shaped.
At intermediate distances E and can
approach time-phase quadrature so that the
E field vector rotates in a plane parallel to the
direction of propagation - Cross field
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THANK YOU!
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