principle of radiation-mechanism of radiation from thin wires

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