lec 1c radiation mechanism & current distribution

7/28/2019 Lec 1c Radiation Mechanism & Current Distribution

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Lec-1c

Radiation Mechanism


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Single Wire

Basic current

charge relationship

ldIz/dt = lqlaz

Create charge acceleration/deceleration-wire curved,

bent, discontinuous or terminatedPeriodic charge acceleration /

deceleration or time varying

current-charge oscillating in timeharmonic motion

How Radiation is Accomplished

(Examination of Sources of Radiation)


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Single Wire (contd)

Maj Conclusions

If charge is not moving-no current & no

radiation

If charge moving with uniform velocity

No radiation if wire is straight, and infinite in

extent

Radiation if wire is curved, bent, discontinuous or

terminated

If charge is oscillating in time- motion it

radiates even if the wire is straight



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Pulse source connected to an open end wire connected to

ground through discrete load

When initially energized, charges set in motion by electricallines of forcer created by source

When accelerated/decelerated from source end & reflection,

radiated fields are produced

Stronger radiation with broad frequency spectrum if pulsesare shorter or compact duration, while continuous time

harmonic oscillating charge produces ideally radiation of

single frequency determined by frequency of oscillation

External source set the charges in motion & produceassociated field radiated

The charge acceleration due to an exciting electric field and

deceleration of charges at the end of wire due to impedance

discontinuities or smooth responsible for em radiation

Qualitative Understanding of Radiation Mechanism

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Two-Wire

Basic capacitive action Application of potential at outer surface of plate

Alignment of electrons according to the point of application

of potential Straightening and elongation of atoms

Negative charge at the outer surface & positive charge at the

inner surface

Production of E. field lines from one plate to the other

Stress in dielectric

Initial flow of current till atoms of dielectric are not aligned

and finally no current

Magnetic field is contained within the plates



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Two-Wire

Consider voltage source connectedto two wire xmn line

E. Field on application of potential

Associated tangent / proportional

lines of force Displacement of free electrons by

lines with consequent movement of charges and H. field

Associated tangent/proportional magnetic lines of force

Electric lines of force - +ve to

ve charge, infinity/infinity to

ve orform closed loops ot starting or ending on any charge

Encircling magnetic field lines around current carrying conductor-

No magnetic charges

E field lines between conductors exhibit distribution of charges

Sinusoidal source sinusoidal E field with period-relative dense lines



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Two-Wire (contd)

Creation of em waves due to time

varying E & H field along xmn line &

enter antenna with associated electric

charges and currents

Removal of part of ant-creation of free

space waves connecting open ends of

electric lines-dashed

Free space waves are also periodic constant phase point, P0

moves outwardly to P1(/2) in one half period

More speed near ant-speed of light away in space



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Two-Wire (contd)

Creation & travel of free-space waves by prolate spheroid with/2interfocal distance

Same free-space waves for/2 dipole except in the immediate

vicinity of ant for prolate spheroid




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Analogy with Water Waves

Disturbances in water by dropping of pebble

Waves do not extinguish even after disturbance removed

New waves generated if disturbances continued

Same analogy true for em waves created by electronic disturbance

Charges are required to generate em waves but not needed tosustain them




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Consider small dipole ant with negligible time of travel

Lines of force for first qtr (max value)-radial distance /4(three lines)

During next qtr original lines travel additional /4 (total /2)-diminishing of

charge density

Analogy-introducing opposite charges which neutralized charges on

conductors at end of 1st cycle-three lines by opposite charges

Again /4 distancedashed lines

Net result -Three upward/downward arrows Since no net charges on ant-detachment of lines of force to unite again to

form closed loops

Procedure continued for 2nd cycle in opposite direction-process repeated

Detachment of waves from antenna Free space waves


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Basic Quantum Physics

Geometry of Atom

Distribution of electrons in arbitals

Arbitals classified as per elongation

Spin motion of electronsneutral atom

Behavior of atom on application of potential-

consequent tangent H field

Why Tangent E & H Field

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Current Distribution on a

Thin Wire Antenna


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Consider lossless 2-wire xmn line

Movement of charges-creation oftravellingIo/2 along each wire

When current arrives at end of each wire undergoes reflection Creation of standing wave by incident & reflected wave-each wire

Current undergoes 1800 between adjoining half cycle-reversal ofarrows

Radiation from each wire -time varying I & termination

Cancellation of fields-balanced symmetrical having equal &

opposite current in corresponding half cycles-resultantly ideal

non radiating xmn line

Current Distribution


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As sections of xmn line flaredassumed unaltered current

distribution in each arm

However, 2-wires are not close now-no cancellation of fields & netradiation by xmn line

Ultimately flared sections have the geometry of dipole antennaalso called standing wave antenna

If l < -phase of standing wave current in each arm is same

throughout its length (direction of arrow from/towards end) Consequently fields radiated by each arm reinforce each other

Current Distribution


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If d


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Ifl> d


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End


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lec 1c radiation mechanism & current distribution

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