lec 1b introduction
TRANSCRIPT
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Lec-1bAntenna Introduction
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An antenna or aerial is a device for radiating
or receiving electromagnetic waves. There is little fundamental difference between
transmitting and receiving antennas since
very often same antenna is used for bothpurpose as in radar.
Antennas are also defined as a metallic
device (as a rod or wire for radiating orreceiving radio waves)
Antenna
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Since antennas play a very important part in a
comm systems, it is essential to have thorough
understanding of the principles on which they
perform their task efficiently.
In the past ants have been treatedsomewhat differently from the sys in which
they are used.
But in recent years ants are designed alongsidethe system. This is because it is essential to
know the system performance as a whole rather
than that of the antenna alone.
Antenna
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Antenna
Antenna connected to source by transmission
line-coax line/hollow pipe for tranportation
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Antenna-Thevenin Equivalent
Thevenin equivalent of antenna in transmit mode
where
Vg = Ideal generator
Zc = Characteristic Impedance
ZA= Ant represented as loadRL = Conduction and dielectric losses associated with ant
Rr = Radiation Resistance
XA= Imaginary part of impedance associated with ant1/1
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Antenna Basics Ideally total energy transfer
However, practically conduction-dielectric losses &
reflection
Exact evaluation of values not possible Mismatch results into standing waves-rise in temp
Assumed max power transfer under conjugate
matching
Improper design-Xmn line an energy storage device
Large field intensities-arching of xmn line
Losses undesirable feature
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Antenna Basics Minimization of losses low loss cable & reduction of
loss resistance RL
Standing wave reduction by matching ant &
characteristic impedance analogous to matching
loads to xmn line
Similar circuit for ant system in Reception mode-
source replacing generator
Rr in receive mode represented for transfer of energy
from free space In addition to reception / xmn ant used for
optimization/ suppressiondirectional device in
addition to probing device
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Types of Antenna Wire Antennas
Very familiar automobiles, buildings, ships, aircrafts,
spacecrafts etc
Types-straight wire, loop,& helix
Variations in loop
Circular common simplicity of construction
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Types of Antenna Aperture Antennas
HF applications
Suitable for ac, spacecraft-easily flush mounted
Covered by dielectric-hazardous conditions Conical HornPyramidal Horn
Rect. Waveguide
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Microstrip Antennas
Metallic patch on grounded substrate
Suitable for ac, sc satellites, missiles, cars , mobile sets
Rectangular strip Circular
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Array Antennas
Yagi-Uda Slotted Waveguide
Microstrip patch Aperture
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Reflector Antennas
Parabolic Reflector (FF) Cassegrain Feed
Corner Reflector
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Lens Antennas
n>1 Lenses
n
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End