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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)
7/28/2019 Lec 1c Radiation Mechanism & Current Distribution
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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
How Radiation is Accomplished
(Examination of Sources of Radiation)
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7/28/2019 Lec 1c Radiation Mechanism & Current Distribution
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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
How Radiation is Accomplished
(Examination of Sources of Radiation)
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7/28/2019 Lec 1c Radiation Mechanism & Current Distribution
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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
How Radiation is Accomplished
(Examination of Sources of Radiation)
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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
How Radiation is Accomplished
(Examination of Sources of Radiation)
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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
How Radiation is Accomplished
(Examination of Sources of Radiation)
7/28/2019 Lec 1c Radiation Mechanism & Current Distribution
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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
How Radiation is Accomplished
(Examination of Sources of Radiation)
7/28/2019 Lec 1c Radiation Mechanism & Current Distribution
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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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