practical antenna
TRANSCRIPT
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Practical Antennaand
Methods of Excitations
Practical consideration
relating to feeding and
matching of antennas
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Effect of earth on vertical pattern
When antenna is isolated and close to earth, earth
has considerable effect on pattern.
This is significant due to the flow of current in earth
which modifies the pattern. Magnitude and phase of this current depend on,
frequency, conductivity and permeability of the
earth.
Earth is perfect conductor at low and medium
frequencies.
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Vertical and Horizontal antenna
Direct and reflectedrays changes theimage charge andcurrents are shown.
The actual andimage antenna havecharges of oppositepolarity. In case ofvertical antenna,
currents are in samedirection. But inhorizontal antennacurrents are inopposite direction.
Fig. Actual and image antenna
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Vertical Antenna
Fig. Vertical pattern of vertical antenna above the earth
When vertical antenna is above the earth then if antenna
height is h from the earth, its image height will be 2h.
Resultant field pattern is product of the antenna and the
vertical pattern of two uniformly excited non directionalradiators spaced at h.
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Horizontal Antenna
For horizontal antenna, vertical pattern is product of
vertical pattern of antenna and vertical pattern of
uniformly excited non directional radiator spaced at
lemda.
Radiation pattern of horizontal antenna above the earth
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Methods of excitation of antenna
Main four type methods for excitation:
a) Center-fed
b) Shunt-fed/ Delta matched excitation
c) End-fed
d) Tapped-fed
Good Impedance
matched
Standing wave can
be eliminated
Simplest method
Improper
impedance matching
High VSWR
Lower impedance
point
Reduces radiation
by wires
Standing wave
reduce as
mismatch reduced
Dipole is fed at
center by open-wire
transmission line
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Impedance Matching Techniques
Impedance matching can be done by using
1. Stubs
2. Folded dipole
3. Baluns
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Stubs
A stubs is a piece of transmission line one end of which is connected to
the transmission line for matching with the load impedance. Its purpose is to be tuned out the reactance.
Second end of stubs can be open circuited or short circuited.
Short circuited lines are used to avoid radiation losses.
Stub matching of transmission lines
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Baluns
The word derived from the BALanced to UNbalanced.
A dipole is a balanced load. In a dipole two arms haveequal current. The impedance of the arms to groundshould be same.
A balun is a balanced to unbalanced transformer.
It is a circuit element connected between a balanced lineand unbalanced line or an antenna.
As a coaxial line is unbalanced balun is connected betweenthe coaxial line and dipole.
Transmission Lines are used as a baluns at high frequenciesand center tapped transformer is used at lowerfrequencies.The primary is unbalanced. Center tappedsecondary winding is balanced and is connected to anantenna.
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Baluns are of two types.1. Broad band :Example: Conventional Transformer
2. Narrow band
Example : choke
Broad band balun Narrow band balun
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Presented By: Ms. Shail Bhatt