Introduction to Antenna Fundamentals

7/24/2019 Introduction to Antenna Fundamentals

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Set III. Dr. Zuhair M. Hejaz Antennas and Wave Propagation1

Introduction to Antenna Fundamentals

A Review

How an Antenna Radiates

In order to know how an antenna radiates, let us first

consider how radiation occurs. A conducting wire

radiates mainly because of time-varying current or an

acceleration (or deceleration) of charges. If there is no

motion of charges in a wire, no radiation takes place,

since no flow of current occurs. Radiation will not occur

even if charges are moving with uniform velocity along a

straight wire. However, charges moving with uniform

velocity along a curved or bent wire will produce

radiation. If the charge is oscillating with time, then

radiation occurs even along a straight wire.

If a source is connected to a transmission line with a

sinusoidal voltage applied across the transmission line,


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an electric field is created which is sinusoidal in nature

and results in the creation of electric lines of force. The

free electrons on the line are forcibly displaced by the

electric lines of force and the movement of these charges

causes the flow of current which in turn leads to the

creation of a magnetic field. In turn it creates electric

field and so on as long as the source alternating

voltage is sustained.

The wave transition into free space is show below.


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Due to the time varying electric and magnetic fields,

electromagnetic waves are created and travel along the

line until they approach open space, free space waves are

formed by connecting the open ends of the electric lines

in closed loops.

Consider a two-wire transmission line (balanced). the

direction of current flow is always opposite so the fields


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strengthen one other between the conductorsbut cancel

each other out away from the conductors. (See below).

If a parallel wire transmission line is left open, then

electric and magnetic fields do escape from the end of

the wire and radiate into space. This is rather inefficient

because most of the incident energy is reflected and a

great deal of cancellation of the electric and magnetic

fields still occurs.


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However, if the last quarter-wavelength (/4) of each

conductor is turned at a right angle then:

The electric fields spread out from the conductor.

The magnetic fields reinforce one another.

This results in efficient radiation.

Such arrangement is called a /2Dipole.


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The intensity/strength of the radiated power Prad(related

to the cross product EH) as a function of direction is

called radiation pattern. For the above dipole, it looks

like a doughnut. See below.


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Spherical Coordinate System

Azimuth and Elevation: These are angles used to

describe a specific position in an antenna's radiation

pattern. Azimuth is a horizontal angle ranging from 0 to

360 degrees or . The elevation angle is a vertical

angle, ranging from 0 degrees (horizon) to 90 degrees.

The radius vector of the distance r from the antenna to

an infinitesimal surface area Ais denoted as

r .

Or it can be presented for a point in space as:

d

d


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Theser , and are commonly termed (spherical polar

co-ordinates) which allows us to specify any radiation

direction uniquely (individually).

,,r are unit vectors

of the coordinates.

The zaxis is taken to be the vertical direction and the xy

plane is horizontal. The

denotes the elevation angle

and denotes the azimuth angle. The xz plane is the

elevation plane ( = 0 ) or the E-plane which is the

plane containing the electric field vector and the

direction of maximum radiation. The xy plane is the


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azimuth plane ( = /2 ) or the H-plane which is the

plane containing the magnetic field vector and the

direction of maximum radiation.

Near and Far Field Regions

The field patterns, associated with an antenna, change

with distance and are associated with two types of

energy: - radiating energy and reactive energy. Hence,

the space surrounding an antenna can be divided into

three regions as shown below.

Field regions around an antenna


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1.

Reactive Near-Field RegionR1: In this region, the

reactive field dominates. The reactive energy

oscillates towards and away from the antenna, thus

appearing as reactance. In this region, energy is only

stored and no energy is dissipated. The outermost

boundary for this region is at a distance

Where R1is the distance from the antenna surface,

D is the largest dimension of the antenna and is

the wavelength.

2.

Radiating Near-Field Region (also called Fresnel

region): This is the region which lies between the

reactive near-field region and the far field region.

Reactive fields are smaller in this field as compared

to the reactive near-field region and the radiation

fields dominate. In this region, the angular field


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distribution is a function of the distance from the

antenna.The outermost boundary for this region is

where R2is the distance from the antenna surface.

3.

Far-field region (also called Fraunhofer region):

The region beyond (R3> R2)

3 > 22/

is the far field region. In this region, the reactive

fields are absent and only the radiation fieldsexist.

The angular f ield distr ibution is not dependent on

the distance from the antenna in

this region and the

power density varies as the inverse square of the

radial distance 21r

.


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Plane and Spherical Waves

Plane waves: The front is a plane (see plot below)

Spherical waves: The front is spherical.

However, at a large distance in the far field region

from the source, the phase front becomes locally a

plane (seen by a receiver)as shown in the plots below).


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Radian and Steradian (Definition)

In a plane angle, the measure is radian (rad) and is

defined from the arc length l and radiusr

of the circle as: rl/

So if rl , 1 rad

l

r


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In a solid angle the measure is steradian (sr) and is

defined for a sphere from the area of the arc Aand2

r of

the radius as shown below:

2/ rA .

If2

rA , then srsteradian1

Infinitesimal Area is equivalent to an area of a square2

r

The area of a sphere of radius ris given by

so in a closed sphere there are 2/ rA sr

steradians (sr)


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For a sphere of radius r, an infinitesimal surface area dA

is given as:

and hence the element of solid angle of a sphere is

given from2

/ rA by:

This formula is of prime importance for calculation of

beam solid angle, Radiated power, directivityand gain

of an antenna which will be discussed later.

Other Classification of Antennas by Size

Let l be the antenna dimension:

1.

Electrically small, l

Primarily used at low frequencies where the

wavelength is long as in telephony, broadcasting,

marine and navigation.


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2.

Resonant antennas, 2/l

Most efficient; examples are slots, dipoles, patches and

have a wide range of communication applications in the

HF band, beamed arrays of dipoles.

3.

Electrically large, l

Can be composed of many individual resonant antennas;

good for radar applications (high gain, narrow beam, low

side-lobes).

Other Terms

Reciprocity

Reciprocityis an antenna ability to transfer energy from

free space to its receiver with the same efficiency with

which it transfers energy from the transmitter into free

space. Or in short: Antennas exhibit the same radiation

pattern for transmission and reception (see Fig. below).


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Polarization

The polarizationof an antenna refers to the direction of

the electric field it produces with reference to the Earths

surface. It can be linear (horizontal/vertical), circular, or

elliptical (right hand polarization or left hand

polarization). See figs below.

Polarization is important because the receiving antenna

should have the same polarization as the transmitting

antenna to maximize received power.

Example of a half-dipole antenna- verticallypolarized.


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Example of a circularly polarized radiation is

produced by helical antennas

In general, the common types of polarizations can be

summarized as shown below.

Introduction to Antenna Fundamentals

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