B.Tech Electronics & Communication Engineering, Semester VI

INSTITUTE OF TECHNOLOGY

NIRMA UNIVERSITY

ANTENNAS AND WAVE PROPAGATION

EC602

2/4/20161

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

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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

Aarti Gehani

Asst. Prof.

Institute of Technology, Nirma University

(aarti.gehani@nirmauni.ac.in)

CH:-1

BASIC ANTENNA CONCEPTS AND

THEOREMS

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Topics

Introduction

How does an antenna radiate?

Various definitions

Antenna parameters

Transmission formula

Various theorems

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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

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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HPBW & FNBW

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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

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

DIRECTIVITY D

DIRECTIVITY D (CONTI.)

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)

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

GAIN G(CONTI.)

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

DIRECTIVITY

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

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.

2/4/201649

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.

pdf

2/4/201651