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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