antennas and wave propagation ec602 · pdf filek.d. prasad -antennas & wave propagation,...
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B.Tech Electronics & Communication Engineering, Semester VI
INSTITUTE OF TECHNOLOGY
NIRMA UNIVERSITY
ANTENNAS AND WAVE PROPAGATION
EC602
2/4/20161
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Lesson Planning (L-3,P-2,C-4)
Chapter
No.
Name Hours
1. Basic antenna concepts and
theorems
3
2. Point sources and Arrays 11
3. Electric dipole and thin layer
antennas
5
4. Loop Antenna 4
5. Yagi-Uda Antenna 1
6. Helical antenna 2
7. Reflector Antennas 4
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Chapter
No.
Name Hours
9. Slot horn & complementary
antennas
2
10. Patch Antenna 3
11. Antennas for special applications 4
12. Antennas measurement 3
13. Radio wave propagation 3
2/4/20163
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Text/Reference Book
1. J. D. Krauss - Antennas, McGraw Hill
2. K.D. Prasad - Antennas & Wave Propagation, Satyaprakash
Publications
3. Jordan & Balmain - Electromagnetic wave & radiating
systems, PHI Publication
4. C. A. Balanis- Antenna Theory, Analysis & Design, Wiley
India Pvt. Ltd.
2/4/20164
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Aarti Gehani
Asst. Prof.
Institute of Technology, Nirma University
CH:-1
BASIC ANTENNA CONCEPTS AND
THEOREMS
2/4/20165
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Topics
Introduction
How does an antenna radiate?
Various definitions
Antenna parameters
Transmission formula
Various theorems
2/4/20166
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INTRODUCTION
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INTRODUCTION (CONTI.)
• An antenna is considered as a region of transition
between a transmission line and space.
• Antenna converts electrons to photons, or vice versa.
• Antenna radiate/couple/concentrate/direct
electromagnetic energy in the desired/assigned
direction.
• No hard and fast rule for selecting antenna
Antenna
Isotropic/Omni-directional/Non-directional
Anisotropic/Directional
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INTRODUCTION (CONTI.)• Some application (e.g., radars, mobile), same antenna for
transmission and reception
• Some application (e.g., radio, television), separate antennafor transmission and reception
• No difference in selection factors for transmitting andreceiving antenna
• Cost, size and shape, etc. important
• High efficiency and high gain- basic requirement fortransmitting antenna
• Low side lobes and large SNR- basic requirement forreceiving antenna
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BASIC ANTENNA PARAMETERS• Radiation is produced by accelerated or decelerated charge.
• Basic radiation equation:
IL=Qv (A m s-1)
I = time changing current, As-1
L = length of current element, m
Q = charge, C
v = time change of velocity which equals theacceleration of the charge, m s-2
• So, time changing current and accelerated chargeradiates
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HOW DOES ANTENNA RADIATES?
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• Electric field comes into picture
• i.e. change in voltage is only possibility
• So, voltage rise at OC.
• Current starts flowing back
• After some time, voltage also starts flowing back.
• If line is SC, voltage = 0 and vice versa
• In case of perfect OC or SC, theoretically perfect reflection
• Waves takes some time to change their direction
• Meanwhile some energy leaks in to the space
• This leakage is called radiation.
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The more is the opening, the more time wave takes to
change the direction, thus more energy leakage in to the
space.
Maximum radiation when ends are flared to make 180°
angle
Thus, an antenna is a transition device, or transducer,
between guided wave and a free space wave or vice
versa.
Antenna is a device which interfaces circuit and space.
Circuit point of view, antenna is a resistance Rr, called the
radiation resistance.
Not a physical resistance, but a resistance coupled from
space to antenna terminal2/4/201615
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PATTERN
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PATTERN (CONTI.)
Radiation pattern or Antenna pattern is defined as “a
mathematical function or a graphical representation of the
radiation properties of the antenna as a function of space
coordinates”.
Radiation pattern is determined in far field region.
Radiation properties include power flux density, radiation
intensity, field strength and polarization.
Pattern are of two types:
1. Power Pattern
2. Field Pattern
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PATTERN (CONTI.)
Power Pattern: normalized power vs. spherical coordinate
position [3]
OR
Power Pattern: Trace of received power at constant radius
[2]
Field Pattern: normalized |E| or |H| vs. spherical
coordinate position [3]
OR
Field Pattern: Graph of spatial variation of the electric (or
magnetic) field along a constant variation [2]
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PATTERN (CONTI.)
Various parts of a radiation patterns are called as lobes,
which are further classified as major or main, minor, side
and back lobes.
Major lobe: The radiation lobe containing the direction
of maximum radiation
Minor lobe: Lobe excepting major lobe
Minor lobe = Side lobe + Back lobe
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PATTERN (CONTI.)
Side lobe: Radiation lobe in any direction other than the
intended lobe
Back lobe: Radiation lobe whose axis makes an
approximately 180° with respect to main lobe
Minor lobes represent the radiation in undesired
directions and they should be minimized.
Level of minor lobes is usually expressed as a ratio of
power density in the lobe in question to that of the major
lobe.
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2/4/201621
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2/4/201622
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HPBW & FNBW
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2/4/201624
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HPBW & FNBW (CONTI.)
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EXAMPLE-1
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RADIAN, STERADIAN & BEAM AREA/
BEAM SOLID ANGLE ΩA
The measure of a plane angle is a radian.
One radian is defined as the angle with its vertex at the
center of a circle of radius r that is subtended by an arc whose
length is r.
Since the circumference of a circle of radius r is 2πr, there
are 2π rad (2πr/r) in a full circle.
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RADIAN, STERADIAN & BEAM AREA/
BEAM SOLID ANGLE ΩA (CONTI.)
The measure of solid angle is steradian.
One steradian is defined as the solid angle with its vertex at
the center of a sphere of radius r that is subtended by a
spherical surface area equal to that of a square with each side
of length r.
Since the area of a sphere of radius r is A=4πr2, there are 4πsr (4πr2/r2) in a closed sphere.
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RADIAN, STERADIAN & BEAM AREA/
BEAM SOLID ANGLE ΩA (CONTI.)
The infinitesimal area dA on the surface of a sphere of radius
r, is given by
dA=(r dθ) (rsinθ dφ) = r2 dΩ
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RADIAN, STERADIAN & BEAM AREA/
BEAM SOLID ANGLE ΩA (CONTI.)
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RADIAN, STERADIAN & BEAM AREA/
BEAM SOLID ANGLE ΩA (CONTI.)
Beam area is the solid angle through which all of power
radiated by the antenna would stream if P(θ,φ) maintained
its maximum value over ΩA and was zero elsewhere. Thus the
power radiated = P(θ,φ) ΩA watts.
It can also be described approximately as
Beam area≌ ΩA ≌ θHPφHP (sr)
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RADIATION INTENSITY
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BEAM EFFICIENCY
Total beam area = Main beam area + Minor lobe area
Therefore, ΩA= ΩM+ Ωm
Ratio of main beam area to total area is called the beam
efficiency εM. Thus,
Beam efficiency = εM = ΩM/ ΩA (dimensionless)
Ratio of minor lobe area to the total beam area is called
stray factor.
εm = Ωm/ ΩA
It follows that εM+ εm =1
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DIRECTIVITY D
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DIRECTIVITY D (CONTI.)
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DIRECTIVITY D (CONTI.)
Thus, the directivity is the ratio of the area of a sphere (4π sr) to the beam area ΩA of the antenna.
Smaller the beam area, larger the dirctivity D
The idealized isotropic antenna (ΩA = 4π sr) has the lowest possible directivity D=1
Unit of directivity is dBi (decibels over isotropic i.e. wrt isotropic antenna)
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GAIN G
Gain G of an antenna is a quantity which is less than the
directivity D due to ohmic losses in the antenna.
In transmitting, losses involve power fed to the antenna
which is not radiated but heats the antenna structure.
Mismatch in feeding also reduces gain.
Ratio of gain to directivity is called antenna efficiency
factor.
G = kD
k = efficiency factor (0 ≤ k ≤ 1), dimensionless
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GAIN G(CONTI.)
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ANTENNA APERTURES
The aperture of an antenna is the area that captures energy
from a passing radio wave.
P=SA (W)
Effective aperture is not always equal to the physical aperture
Aperture efficiency is given by
εap = Ae/Ap (dimensionless)
For horn and parabolic reflector antenna, aperture
efficiencies are in the range of 50 to 80%.
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ANTENNA APERTURES (CONTI.)
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ANTENNA APERTURES (CONTI.)
Equating both the equations and substituting Er = EaAe/rλ,
we get
λ2 = Ae ΩA (m2) Aperture-beam-area-relation
We also know that D = 4π/ΩA, so from the above equation
we can say that
D = 4π Ae / λ2 Directivity from Aperture
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DIRECTIVITY
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RADIATION RESISTANCE
Rr is that part of an antenna’s feed point resistance that is
caused by radiation of electromagnetic waves from the
antenna.
Determined by the geometry and not the material
Equivalent to a resistor in a circuit
Caused by the radiation reaction of the conduction of
electrons in antenna.
Accelerated electrons produces EM waves. Waves carries
energy taken from electrons.
Loss of energy of the electrons appears as an effective
resistance to movement of other electrons.2/4/201644
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Antenna Field Zones
Mainly two types: Near field or Fresnel zone and Far field or
Fraunhofer zone.
Boundary between the two is arbitrarily taken as
R = (2L2)/λ (m); L = maximum dimension of the antenna
(m) and λ = wavelength (m)
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RADIO COMMUNICATION LINK
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RADIO COMMUNICATION LINK (CONTI.)
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SOME MORE DEFINITIONS
Polarization: Orientation of E-field vector of a wave
Axial Ratio: Ratio of major to minor axes of polarization
ellipse
Signal to Noise Ratio: Ratio of signal fed to the network
to the noise
Antenna Temperature: Fictitious temperature at the input
of an antenna which would account for noise ∆N at the
output
Front to Back Ratio: Ratio of energy in main lobe to that
in the back lobe
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SOME MORE DEFINITIONS (CONT.)
Antenna Bandwidth: Range of frequency over which an
antenna maintains its certain required characteristics, viz.
gain, radiation resistance, polarization, FBR, etc.
Driving Point or Terminal Impedance: Impedance
measured at the input terminals of an antenna.
Effective Height/ Length: Ratio of induced voltage at the
terminal of the receiving antenna under an open circuit
condition to the electric field intensity or strength.
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Antenna Theorems
1. Equality of directional patterns: The directional
pattern of a receiving antenna is identical with its
directional pattern as transmitting antenna.
2. Equality of transmitting and receiving antenna
impedance: The impedance of an isolated antenna when
used for receiving is the same as when used for
transmitting.
3. Equality of effective length: The effective length of an
antenna for receiving is equal to its effective length as a
transmitting antenna.
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REFERENCES
1. J. D. Krauss - Antennas, McGraw Hill
2. C. A. Balanis- Antenna Theory, Analysis & Design, Wiley
India Pvt. Ltd.
3. http://www.ece.msstate.edu/~donohoe/ece4990notes2.
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