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This is a brief presentation to explain principle of radiation and mechanism of radiation from halfwave dipole, folded dipole and quarterwave monopole, all viewed as extensions of two-wire transmission line. Main reference: "Antenna Theory", CA Balanis, Wiley-Interscience
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1Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011 1
Principles of Radiation: Mechanism of
Radiation from Thin Wires
(using txn-line concepts)
2Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011 2
Radiation is the net
outward flow of
electromagnetic energy
from source
Radiated power is carried
by free space waves, with
EM fields detached from
source
Accelerating (or decelerating) electric charges
must radiate (Classical theory of Radiation)
Radiation
3Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011 3
Sources of Radiation
Hot Objects (charged particles in rapid motion)
Stars (basis of astronomy)
Collisions between charged particles (braking radiation ‘Bremsstrahlung’)
Charged particles travelling faster than speed of light (Cherenkov radiation)
Electrons in atoms jumping from higher to lower energy states (Quantum theory)
Time-varying currents in conductors (Linear, Surface and Volume Current Elements)
4Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011 4
Alternating Current Elements:
The Basic Building Blocks of Antennas
Linear Current Element
Surface Current Element
Volume Current Element
I dl
sJ ds
J dv
5Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011 5
Mechanism of Radiation from Thin Wires
Equivalence between Moving Charge and Current
Element
Fundamental Equation of Radiation from Thin Wires
dQ v = I dL
dQ (dv/dt) = (dI/dt) dL
6Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011 6
Radiation from Single Wires
Steady current, straight wire – No RadiationTime-harmonic current – Radiation at the frequency of oscillationPulsed current – Radiation over broad frequency spectrumCurrent in curved, bent, discontinuous, terminated or truncated wire – Causes radiation
a) Curved b)bent c) discontinuous
d) Terminated e) truncated
Radiation Mechanism: 2-Wire Configuration [1/4]
Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011
Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011
Radiation Mechanism: 2-Wire Configuration [2/4]
Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011
Radiation Mechanism: 2-Wire Configuration [3/4]
Imagine the charge pattern on conductors travelling right withthe voltage wave. E field lines initially attached to charges onthe conductors form closed loops once every half cycle atmoments of charge polarity reversal. Free space wave thusformed is not attached to the line anymore and continues toexist even if source is switched off.
Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011
Radiation Mechanism: 2-Wire Configuration [4/4]
11Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011 11
12Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011 12
Radiation from λ/2 - Dipole
13Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011 13
Radiation from a vertical 3λ/2 Dipole
14Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011 14
Dipole Antenna
15Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011 15
Half-Wave Dipole Antenna
16Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011 16
/2 folded dipole = two centre-fed /2 dip oles,
co-located
17Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011 17
Half-Wave Folded Dipole Antenna
18Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011 18
19Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011 19
Monopole Antenna
Ground Plane
20Presented by APN Rao, Dept ECE, GRIET, Hyderabad. Jul 2011 20
Monopole Antenna
