Lect Antenna Chap 02 i

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EE-402 WAVE PROPAGATION AND ANTENNAS

1



Chapter 02

Fundamental Parameters of Antennas

“Antenna Theory, Analysis and design ”, C. A Balanis, 3 rd Edition, John wiley & sons, inc., publication 2005,United States of America



Radiation Pattern• A mathematical function or a graphical representation of

the radiation properties of the antenna as a function ofspace coordinates

is represented as a function of the directionalcoordinates

• Radiation properties include power flux density, radiationintensity, field strength, directivity, phase or polarization



Radiation Pattern

Coordinate system for antenna analysis



Radiation Pattern• Often the field and power patterns are normalized with

respect to their maximum value, yielding normalized field and

power patterns• Power pattern is usually plotted on a logarithmic scale or

more commonly in decibels (dB)

Field attern in linear scale t icall re resents a lot of the

magnitude of the electric or magnetic field as a function of theangular space.

Power pattern(in linear scale) typically represents a plot of the

square of the magnitude of the electric or magnetic field as a

function of the angular space.

Power pattern(in dB) represents the magnitude of the electricor magnetic field in decibels, as a function of the angular

space



Radiation Pattern

Field Pattern (in linear scale) Power Pattern (in linear scale)



Radiation Pattern

Power Pattern (in dB)



Radiation Pattern

• Half-power (−3 dB points), relative to the maximum value of

the pattern:

Field pattern at 0.707 value of its maximum

.maximum

Power pattern (in dB) at −3 dB value of its maximum

• Angular separation between the two half-power points isknown as Half power beamwidth (HPBW)



Radiation Pattern Lobes

Radiation lobes and beam-widths of an antenna pattern




Linear plot of power pattern and its associated lobes and beam-widths




• A radiation lobe is a “portion of the radiation pattern bounded

by regions of relatively weak radiation intensity”• A major lobe (also called main beam) is defined as “the

radiation lobe containing the direction of maximum radiation”

• A side lobe is “a radiation lobe in any direction other than theintended lobe.” (Usually a side lobe is adjacent to the main

lobe)

• A back lobe is “a radiation lobe whose axis makes an angle ofapproximately 180 with respect to the beam of an antenna”




• Minor lobes usually represent radiation in undesireddirections, and they should be minimized

• Side lobes are normally the largest of the minor lobes

• The level of minor lobes is usually expressed as a ratio of the

power density in the lobe relative to that of the major lobe.This ratio is often termed the side lobe ratio or side lobe level

• In most radar systems, low side lobe ratios are very importantto minimize false target indications through the side lobes




• For an amplitude pattern of an antenna, there would be, ingeneral, three electric-field components (Er ,Eθ ,Eφ)

• In the far field, the radial Er component for all antennas iszero or vanishingly small compared to either one, or both, of

t e ot er two components• In general, the magnitude of the total electric field would be:



Isotropic, Directional and Omni-directional

Patterns

• An isotropic radiator is defined as “a hypothetical losslessantenna having equal radiation in all directions”

• A directional antenna is one “having the property of radiatingor receiving electromagnetic waves more effectively in some

rect ons t an n ot ers• Omni-directional pattern is defined as one “having an

essentially non-directional pattern in a given plane and adirectional pattern in any orthogonal plane

• An omni-directional pattern is the a special type of adirectional pattern




Patterns

Principle E- and H-plane patterns for a pyramidal horn antenna




Patterns

Omni-directional antenna pattern



Principle Patterns

• The E-plane is defined as “the plane containing the electric-

field vector and the direction of maximum radiation”

• The H-plane as “the plane containing the magnetic-fieldvector and the direction of maximum radiation”



Radian and Steradian

• The measure of a plane angle is a radian

• One radian is defined as the plane angle with its vertex at the

center of a circle of radius r that is subtended by an arc whoselength is r

• The circumference of a circle of radius r is C = 2πr,so, thereπ π

• The measure of a solid angle is a steradian

• One steradian is defined as the solid angle with its vertex at thecenter 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• The area of a sphere of radius r is A = 4πr2,so,there are 4π sr

(4πr2 /r2) in a closed sphere




Radian Steradian




• The differential area dA on the surface of a sphere of radius r isgiven by:

• Therefore, the element of solid angle dΩ of a sphere can bewritten as:

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Lect Antenna Chap 02 i