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