Lec 1a Introductory Lec

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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Lec 1a Introductory Lec