lec 1a introductory lec
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7/29/2019 Lec 1a Introductory Lec
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Lec-1a
Introductory Lec
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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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Movement of Charge
Movement of electrons in bands
Absorption/release of packet of energy
Movement of energy across conductor
Flow of current
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Flow of Current LF (DC)
Flow of current analogous to flow of water
Flow of energy for dc vis a viz diameter of
conductor
For LF flow through centre of conductor
No skin effect for dc
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Flow of Current HF (AC)
Skin effect for HF-No current through centre ofconductor
Higher the freq more pronounced skin effect
For LF flow through Centre of conductor Hollow conductors for HF
Shortening of lines of force at HF-Ends of field
More thickness of skin for high energy carriage Insufficient conductor thickness -Standing waves
Lesser skin effect for LF
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Impedance Matching- Max Power Transfer
For max power transfer load impedance should bematched to line impedance
Any mismatch results into reflection of energy
Example In either of cases lesser power transferred
Reactance nullified-Conjugate matching
If not nullified-Reflection/Temperature rise Antenna as load
Complete matching not possible
Use of antenna tuners
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Power Three Types
Apparent Simple voltage current relationship
VI
Real/True VI Cos
Max value when not used
Decays on use
Used for consumption
Reactive VI Sin
Min when not used-vice versa
Used for storage
Power FactorPower factor (PF) = Real Power = VI Cos = Cos
Apparent Power VI
PF = Cos
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Transmission Line Parameters
Distributed Parameters
Parameters R, L C distributed along the line
Lump
Equivalent represented in a circuit
Characteristic Impedance
All three represented by a single impedance Z
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End
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