single dipole and n-array antenna

8/12/2019 Single Dipole and N-Array Antenna

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DEPARTMENT OF ELECTRICAL AND COMPUTER

ENGINEERING

ELECTRONICS AND COMMUNICATION STREAM

PROJECT TITLE: PERFORMANCE MEASUREMENT AND

COMPARISON OF SINGLE DIPOLE ANTENNA AND

N-ARRAY ANTENNA

GROUP MEMBERS ID NO.1. BRHANE DADISO CET/UR0146/012. HAGOS GEBRETSADIK CET/UR0279/01

3. SIMRET KEBEDE CET/UR0493/014. EDEN TEWELDE CET/UR0180/01

SUBMITTED TO: TEKLE B. (Msc.)SUBMISSION DATE:


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Table of content

Abstract

----------------------------------------------------------------------------------------------

Acknowledgment

-----------------------------------------------------------------------------------

Introduction

-------------------------------------------------------------------------------------

Background

---------------------------------------------------------------------------------------

Chapter 1

1. Basic Antenna parameters Antenna

---------------------------------------------------------------------------1. Introduction

----------------------------------------------------------------------------------------------2. Radiation pattern lobes

----------------------------------------------------------------------------------3. Field Regions

-----------------------------------------------------------------------------------------------4. Radiation power density5. Radiation Intensity6. Beam width7. Directivity8. Gain

Chapter 2

2. Single Dipole Antenna


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2.1 Introduction

2.2 Dipole Basic characteristics

2.3 Finite length Dipole

2.3.1 Current Distribution

2.3.2 Radiation fields: Element factor, space factor, pattern multiplication

2.3.3 Power density, Radiation intensity, Radiation resistance

2.3.4 Directivity

2.3.5 Input Resistance

2.4 Applications of Dipole Antenna

Chapter 3

3. Array Antenna

3.1 Introduction

3.2 Basic Array Antenna characteristics

3.3 N-Element Array: Uniform Amplitude $ Spacing

3.4 N-Element Array Directivity

3.4.1 Broad side Array

3.4.2 Ordinary End-fire Array

3.4.3 Hansen wood yard End-fire Array

3.5 Applications of N-Element Array

Chapter 4


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ABSTRACT

This project is the measurement and comparison of single dipole antenna and N-array antenna. In this

project we used the mat lab simulation for the comparison of the directivity of both dipole and N-array

antenna. Also we will include the mathematical expression of the antenna parameters.


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Acknowledgement

First and foremost we would like to thank to our supervisor of this project, Mr. Tekle A. (M.Sc) for her

constant encouragement and support throughout the whole project, especially for the useful comments

given during the project duration.

We also wanted to thank our friends Gebrekirustos Mebrahitom information system department who

give us his laptop to prepare our document and to search data from internet.

And also we thank to all the lab assistance of electrical and computer department for their contribution to

accomplish the project on time.


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IntroductionAn antenna is defined as a usually metallic device (as a rod or wire) for radiating or receiving radio

waves. The IEEE Standard Definitions of Terms for Antennas (IEEE Std 145 1983) defines the

antenna or aerial as a means for radiating or receiving radio waves. In other words the antenna is the

transitional structure between free-space and a guiding device. The guiding device or transmission line

may take the form of a coaxial line or a hollow pipe (waveguide), and it is used to transport

electromagnetic energy from the transmitting source to the antenna or from the antenna to the receiver. In

the former case, we have a transmitting antenna and in the latter a receiving antenna.

Antennas have become ubiquitous devices and occupy a salient position in wireless systems. Radio and

TV as well as satellite and new generation mobile communications have experienced the largest growth

among industry systems. The global wireless market continues to grow at breakneck speed and the

strongest economic and social impact nowadays comes from cellular telephony, personal communications

and satellite navigation systems. All of the above systems have served as motivation for engineers to

incorporate elegant antennas into handy and portable systems.

A device able to receive or transmit electromagnetic energy is called an antenna. The antenna plays

the role of a transitional structure between free space and a guiding device. An antenna consists of one or

more elements. A single-element antenna is usually not enough to achieve technical needs. That happens

because its performance is limited. A set of discrete elements, which constitute an antenna array, offers


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the solution to the transmission and/or reception of electromagnetic energy. The geometry and the type of

elements characterize an antenna array. For simplicity, implementation and fabrication reasons, the

elements are chosen in such a way so as to be identical and parallel. For the same reasons, uniformly

spaced linear arrays are mostly encountered in practice.

BACKGROUNDThe history of antennas dates back to James Clerk Maxwell who unified the theories of electricity and

magnetism, and eloquently represented their relations through a set of profound equations best known as

Maxwells Equations. His work was first published in 1873. He also showed that light was

electromagnetic and that both light and electromagnetic waves travel by wave disturbances of the same

speed. In 1886, Professor Heinrich Rudolph Hertz demonstrated the first wireless electromagnetic system.He was able to produce in his laboratory at a wavelength of 4 m a spark in the gap of a transmitting /2

dipole which was then detected as a spark in the gap of a nearby loop. It was not until 1901 that Guglielmo

Marconi was able to send signals over large distances. He performed, in 1901, the first transatlantic

transmission from Plod in Cornwall, England, to St. Johns Newfoundland. His transmitting antenna

consisted of 50 vertical wires in the form of a fan connected to ground through as park transmitter. The

wires were supported horizontally by a guyed wire between two 60-m wooden poles. The receiving

antenna at St. Johns was a 200-m wire pulled and supported by a kite. This was the dawn of the antenna

era. From Marconis inception through the 1940s, antenna technology was primarily centered on wire

related radiating elements and frequencies up to about UHF. It was not until World War II that modern

antenna technology was launched and new elements (such as waveguide apertures, horns, reflectors)

were primarily introduced. Much of this work is captured in the book by Silver. A contributing factor to

this new era was the invention of microwave sources (such as the klystron and magnetron) with

frequencies of 1 GHz and above. While World War II launched a new era in antennas, advances made in

computer architecture and technology during the 1960s through the 1990s have had a major impact on

the advance of modern antenna technology, and they are expected to have an even greater influence on

antenna engineering into the twenty-first century. Beginning primarily in the early1960s, numerical

methods were introduced that allowed previously intractable complex antenna system configurations to be

analyzed and designed very accurately. In addition, asymptotic methods for both low frequencies (e.g.,

Moment Method (MM), Finite-Difference, Finite-Element) and high frequencies (e.g., Geometrical and

Physical Theories of Diffraction) were introduced, contributing significantly to the maturity of the antenna


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field. While in the past antenna design may have been considered a secondary issue in overall system

design, today it plays a critical role. In fact, many system successes rely on the design and performance of

the antenna. Also, while in the first half of this century antenna technology may have been considered

almost a cut and try operation, today it is truly an engineering art. Analysis and design methods are such

that antenna system performance can be predicted with remarkable accuracy. In fact, many antenna

designs proceed directly from the initial design stage to the prototype without intermediate testing. The

level of confidence has increased tremendously.

Prior to World War II most antenna elements were

of the wire type (long wires, dipoles, helices,

rhombuses, fans, etc.), and they were used either as

single elements or in arrays. During and after World

War II, many other radiators, some of which may have

been known for some and others of which were

relatively new, were put into service. This created a

need for better understanding and optimization of their

radiation characteristics. Many of these antennas were

of the aperture type (such as open-ended waveguides,

slots, horns, reflectors, lenses), and they have been

used for communication, radar, remote sensing, and


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deep

e applications both on airborne and earth-based platforms. Many of these operate in the microwave

region.

Prior to the 1950s, antennas with broadband pattern and impedance characteristics had bandwidths not

much greater than about 2:1. In the 1950s, a breakthrough in antenna evolution was created which

extended the maximum bandwidth to as great as 40:1 or more. Because the geometries of these antennas

are specified by angles instead of linear dimensions, they have ideally an infinite bandwidth. Therefore,

they are referred to asfrequency independent. These antennas are primarily used in the 1010,000 MHz

region in a variety of applications including TV, point-to-point communications, feeds for reflectors and

lenses, and many others.

Major advances in millimeter wave antennas have been made in recent years, including integrated

antennas where active and passive circuits are combined with the radiating elements in one compact unit

(monolithic form).

Chapter 1

1. Antenna parameters

Introduction

1.1 Radiation Pattern LobesA radiation lobe is a portion of the radiation pattern bounded by regions of relatively weak radiation

intensity. lobes, which may be sub classified into major or main, minor, side, and back lobes


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Figure 1. (a) Radiation lobes and beam widths of an antenna pattern. (b) Linear plot of power pattern and

its associated lobes and beam widths.

A normalized three-dimensional far-field amplitude pattern, plotted on a linear scale, of a 10-element linear

antenna array of isotropic sources with a spacing of d=0.25and progressive phase shift =0.6, between

the elements is shown in Figure 2.4. It is evident that this pattern has one major lobe, five minor lobes and

one back lobe. The level of the side lobe is about 9 dB relative to the maximum .

1.2 Field Regions

The space surrounding an antenna is usually subdivided into three regions: (a) reactive near-field, (b)

radiating near-field (Fresnel) and (c) far-field (Fraunhofer) regions as shown in Figure 2.7.


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Figure 2. Field regions of an antenna

Reactive near-field region is defined as that portion of the near-field region immediately surrounding the

antenna wherein the reactive field predominates. For most antennas, the outer boundary of this region is

commonly taken to exist at a distance R


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Electromagnetic waves are used to transport information through a wireless medium or a guiding structure,

from one point to the other. The quantity used to describe the power associated with an electromagnetic

wave is the instantaneous Pointing vector defined as

(1-1)

The Pointing vector is a power density, the total power crossing a closed surface can be obtained by

integrating the normal component of the Pointing vector over the entire surface. In equation form:

(1-2)

For time-harmonic variations of the form we define the complex field E and H which are related

to their instantaneous counterparts and by

(1-3)

(1-4)

Using the definitions of (2-5) and (2-6) and the identity Re[E ] =1/2[ ], (2-3) can be ejwt e eE jwt+E* jwt

written as

(1-5)

The first term of (2-7) is not a function of time, and the time variations of the second are twice the given

frequency. The time average Poynting vector (average power density) can be written as


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(1-6)

A close observation of (2-8) If the real part of (E X )/2 At this point it will be very natural to assume H*

that the imaginary part must represent the reactive (stored) power density associated with the

electromagnetic fields Based upon the definition of (2-8), the average power radiated by an antenna

(radiated power) can be written as

(1-7)

The isotropic radiator is an ideal source that radiates equally in all directions. Although it does not exist in

practice, it provides a convenient isotropic reference with which to compare other antennas. Because of

its symmetric radiation, its Pointing vector will not be a function of the spherical coordinate angles and .

In addition, it will have only a radial component. Thus the total power radiated by it is given by

(1-8)

And the power density by which is uniformly distributed over the surface of a sphere of radius r.

(1-9)

Which is uniformly distributed over the surface of a sphere of radius r.

1.4 RADIATION INTENSITY

Radiation intensity defined as the power radiated from an antenna per unit solid angle. The radiationintensity is a far-field parameter, and it can be obtained by simply multiplying the radiation density by the

square of the distance. In mathematical form it is expressed as

(1-10)


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Where

U=radiation intensity (W/unit solid angle)

W rad =radiation density (W/m2)

The radiation intensity related to the far-zone electric field of an antenna Referring to Figure 2.4, by:

(1-10a)

Where

The total power is obtained by integrating the radiation intensity, as given by (2-12), over the entire solid

angle of 4. Thus

(1-11)

Where d=element of solid angle=sin d d

For anisotropic source U will be independent of the angles and , as was the case for Wrad. Thus

(2-13) can be written as

(1-13)

Or the radiation intensity of an isotropic source as


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(1-14)

1.5 BEAMWIDTH

The beam width of a pattern is defined as the angular separation between two identical points on opposite

side of the pattern maximum. In an antenna pattern, there are a number of beam widths. One of the most

widely used beam widths is the Half-Power Beam width (HPBW), which is defined by IEEE as: In a

plane containing the direction of the maximum of a beam, the angle between the two directions in which

the radiation intensity is one-half value of the beam. This is demonstrated in Figure 2.2. Another

Important beam width is the angular separation between the first nulls of the pattern, and it is referred to

as the First-Null Beam width (FNBW). The beam width of an antenna is a very important figure of merit

and often is used as a trade-off between it and the side lobe level that is, as the beam width decreases,

the side lobe increases and vice versa. In addition, the beam width of the antenna is also used to describe

the resolution capabilities of the antenna The most common resolution criterion states that the resolution

capability of an antenna to distinguish between two sources is equal to half the first-null beam width

(FNBW/2), which is usually used to approximate the half-power beam width (HPBW).

1.6 DIRECTIVITY

Directivity of an antenna defined as the ratio of the radiation intensity in a given direction from the

antenna to the radiation intensity averaged over all directions. The average radiation intensity is equal to

the total power radiated by the antenna divided by 4. If the direction is not specified, the direction of

maximum radiation intensity is implied. The directivity of a non isotropic source is equal to the ratio of its

radiation intensity in a given direction over that of an isotropic source. In mathematical form, using (2-15),

it can be written as

(1-15)

If the direction is not specified, it implies the direction of maximum radiation intensity (maximum

directivity) expressed as


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(1-15a)

D=directivity (dimensionless)

=maximum directivity (dimensionless)Do

U=radiation intensity (W/unit solid angle)

=maximum radiation intensity (W/unit solid angle)Umax

=radiation intensity of isotropic source (W/unit solid angle)Uo

=total radiated power (W)Prad

Partial directivities for any two orthogonal polarizations For a spherical coordinate system, the total

maximum directivity D0 for the orthogonal and components of an antenna can be written as

While the partial directivities D and D are expressed as

(1-15b)

(1-15c)

Where

U =radiationintensity ina givendirectioncontained in field component

U =radiationintensity ina givendirectioncontained in field component


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(Prad) =radiated power in all directions contained in field component

(Prad) =radiated power in all directions contained in field component

1.6 GAINGain of an antenna (in a given direction) is defined as the ratio of the intensity, in a given direction, to the

radiation intensity that would be obtained if the power accepted by the antenna were radiated isotropically.

The radiation intensity corresponding to the isotropically radiated power is equal to the power accepted

(input) by the antenna divided by 4. In equation form this can be expressed as

(1-16)

relative gain, which is defined as the ratio of the power gain in a given direction to the power gain of a

reference antenna in its referenced direction. The power input must be the same for both antennas the

reference antenna is a lossless isotropic source. Thus

(1-16a)

When the direction is not stated, the power gain is usually taken in the direction of maximum radiation The

total radiated power(Prad) is related to the total input power(Pin)by

1-17)

Where is the antenna radiation efficiency (dimensionless), According to the IEEE Standards, gainecd

does not include losses arising from impedance mismatches (reflection losses) and polarization mismatches

(losses).

We define two gains one, referred to as gain (G), and the other, referred to as absolute gain ( ) Using Gabs

(2-47) reduces (2-46a) to


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(1-18)

which is related to the directivity of (2-16) and (2-21) by

(1-19)

In a similar manner, the maximum value of the gain is related to the maximum directivity of (1-16a) and

(2-23) by

(1-19a)

Connection losses are usually referred to as reflections (mismatch) losses, and they are taken into account

by introducing a reflection(mismatch) efficiency er

(1-19b)

Where is the overall efficiency, the maximum Absolute gaineo Gabs

(1-19c)

If the antenna is matched to the transmission line, that is, the antenna input impedance Z in is equal to the

characteristic impedance of the line (=0) then the two gains are equal ( =G)Zc Gabs

For a spherical coordinate system, the total maximum gain G0 for the orthogonal and components of

an antenna can be written,

(1-20)

While the partial gains and are expressed asG G


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(1-21a)

(1-20b)

where

U =radiationintensity ina givendirectioncontained in E field component

U =radiationintensity ina givendirectioncontained in E field component

=total input (accepted) powerpin

Chapter 2

2. DIPOLE antenna

2.1 Introduction

A dipole antenna is a radio antenna that can be made of a simple wire, with a center-fed driven circuit. It

consists of two metal conductors of rod or wire, oriented parallel and collinear with each other (in line with

each other), with a small space between them. The radio frequency voltage is applied to the antenna at the

center, between the two conductors. These antennas are the simplest practical antennas from a theoretical

point of view.

2. FINITE LENGTH DIPOLE2.2.1 Current Distribution

For a very thin dipole (ideally zero diameter), the current distribution can be written,


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to a good approximation, as

(2)

This distribution assumes that the antenna is center-fed and the current vanishes at the end points (z=l/2).

2.2.2 Radiated Fields: Element Factor, Space Factor, and Pattern Multiplication

For the current distribution of (1) the E-and H-fields can be obtained which are valid in all regions (any

observation point except on the source itself). In general, however, this is not the case. Usually we are

limited to the far-field region, because of the mathematical complications provided in the integration of the

vector potential (A). Since closed form solutions, which are valid everywhere, cannot be obtained for

many antennas, the observations will be restricted to the far-field region. This will be done first in order to

illustrate the procedure. In some cases, even in that region it may become impossible to obtain closed form

solutions. The finite dipole antenna of Figure 1 is subdivided into a number of infinitesimal dipoles of length

3z. As the number of subdivisions is increased, each infinitesimal dipole approaches a length dz. For an

infinitesimal dipole of length dz Positioned along the z-axis at z, the electric and magnetic field components

in the far field are given as

(2-1)

(2-2)

(2-3)


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Figure 3: Finite dipole geometry and far-field approximations.

Where R is given by

Or

Using the far-field approximations

(2-4)

Summing the contributions from all the infinitesimal elements, the summation reduces, in the limit, to

integration. Thus

(2-4a)

The factor outside the brackets is designated as the element factor and that within the brackets as the


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space factor. For this antenna, the element factor is equal to the field of a unit length infinitesimal dipole

located at a reference point (the origin). In general, the element factor depends on the type of current and

its direction of flow while the space factor is a function of the current distribution along the source.

The total field of the antenna is equal to the product of the element and space factors. This is referred toas pattern multiplication for continuously distributed sources, and it can be written as

Total field =( element factor) (space factor)

The pattern multiplication for continuous sources is analogous to the pattern multiplication for

discrete-element antennas (arrays).For the current distribution of (1), (2) can be written as

+

(2-4b)

Each one of the integrals in (4-60) can be integrated using

Where

After some mathematical manipulations, (4-60) takes the form of


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(2-4c)

In a similar manner, or by using the established relationship between the and inthe far field as E H

givenby

(where is wave impedance)Zw

The total component can be written asH

(2-5)

3. Power Density, Radiation Intensity, and Radiation ResistanceFor the dipole, the average Poynting vector can be written as:

(2-6)

The normalized (to 0 dB) elevation power patterns, as given by (10) for l =/4,/2,3/4, and are

shown plotted in Figure 4.6. The current distribution of each is given by (4-56). The power

patterns for an infinitesimal dipole l


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should also increase with length. It is found that the 3-dB beam width of each is equal to

As the length of the dipole increases beyond one wave length (l > ), the number of lobes begin to

increase. The normalized power pattern for a dipole with l =1.25isshown in Figure 2. In Figure

3(a) the three-dimensional pattern is illustrated using the software from [5], while in Figure 3(b)

the two-dimensional (elevation pattern) is depicted. For the three-dimensional illustration, a 90

angular section of the pattern has been omitted to illustrate the elevation plane directional pattern

variations. The current distribution for the dipoles with l=/4,/2,,3/2, and 2,asgivenby(1), is

shown in Figure 4.8To find the total power radiated, the average Poynting vector of (4-63) is

integrated over a sphere of radius r. Thus

(2-7)


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Figure 4: Elevation plane amplitude patterns for a thin dipole with sinusoidal current distribution

(l=/50,/4,/2,3/4,).

(2-7a)


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Figure 5.Three- and two-dimensional amplitude patterns for a thin dipole of l =1.25 and sinusoidal

current distribution.


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Figure 6. Current distributions along the length of a linear wire antenna.

(2-8)Where

(2-8a)The radiation resistance can be written as

(2-9)


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Figure 6 is a plot of as a function of l (in wave lengths) when the antenna is radiating into Rr

free-space The imaginary part of the impedance cannot be derived using the same

method as the real part because, as was explained in Section 4.2.2, the integration over a closed

sphere in (4-13) does not capture the imaginary power contributed by the transverse component

of the power density.W

Figure 7: Radiation resistance, input resistance and directivity of a thin dipole with sinusoidal

current distribution

The imaginary part of the impedance, relative to the current maximum, is given by or

(2-10)4. Directivity

As was illustrated in Figure 1, the radiation pattern of a dipole becomes more directional as its

length increases. When the overall length is greater than one wave length, the number of lobes

increases and the antenna loses its directional properties. The parameter that is used as a figure

of merit for the directional properties of the antenna is the directivity. The directivity was defined

mathematically by:


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(2-11)Where F(, )is related to the radiation intensity U by

(2-12)the dipole antenna of length l has

(2-13)And

Because the pattern is not a function of , the equation reduces to:

The ff Equation can be written, using the above equations, as

Where

(2-14)The maximum value of F( ) varies and depends upon the length of the dipole. Values of the

directivity, as given by the above equations, have been obtained for0


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(2-15)5. Input Resistance

In the previous Section the input impedance was defined as the ratio of the voltage to current at a

pair of terminals or the ratio of the appropriate components of the electric to magnetic fields at a

point. The real part of the input impedance was defined as the input resistance which for a

lossless antenna reduces to the radiation resistance, a result of the radiation of real power. The

radiation resistance of a dipole of length l with sinusoidal current distribution, is referred to the

maximum current which for some lengths (l =/4, 3/4, , etc.) does not occur at the input

terminals of the antenna (see Figure 5). To refer the radiation resistance to the input terminals of

the antenna, the antenna itself is first assumed to be lossless (RL=0). Then the power at the inputterminals is equated to the power at the current maximum.

(2-16)Where

=radiation resistance at input (feed) terminalsRin =radiation resistance at current maximum Eq. (4-70)Rr =current maximumIo =current at input terminalsIin

For a dipole of length l, the current at the input terminals ( ) is related to the current maximum, Iin

by

(2-17)


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Figure 8 Current distribution of a linear wire antenna when current maximum does not occur at the input

terminals. Thus the input radiation resistance can be written as

To compute the radiation resistance (in ohms), directivity (dimensionless and in dB), and input resistance

(in ohms) for a dipole of length l, a MATLAB and FORTRAN computer program has been developed.

The radiated power Prad is computed by numerically integrating (over a closed sphere) the radiation

intensity .The length of the dipole (in wavelengths) must be inserted as an input. When the overall length

of the antenna is a multiple of (i.e., l =n, n=1,2,3,...), it is apparent from (1) and from Figure 5 that Iin

=0. That is

Which indicates that the input resistance at the input terminal , is infinite. In practice this is not the

casebecause the current distribution does not follow an exact sinusoidal distribution, especially at the feed


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point. It has, however, very high values. Two of the primary factors which contribute to the non sinusoidal

current distribution on an actual wire antenna are the nonzero radius of the wire and finite gap spacing at

the terminals. The radiation resistance and input resistance, are based on the ideal current distribution and

do not account for the finite radius of the wire or the gap spacing at the feed. Although the radius of the

wire does not strongly influence the resistances, the gap spacing at the feed does play a significant role

especially when the current at and near the feed point is small.

Common applications of dipole antenna

Dipole Antennas are most widely used antennas for wireless mobile communication systems . An array of

dipole elements is extensively used as an antenna at the base station of a land mobile system while the

monopole, because of its broadband characteristics and simple construction, is perhaps to most common

antenna element for portable equipment, such as cellular telephones, cordless telephones, automobiles,

trains, etc.

An antenna configuration that is widely used as a base-station antenna for mobile communication and is

seen almost every where. It is a triangular array configuration consisting of twelve dipoles, with four

dipoles on each side of the triangle. Each four-element array, on each side of the triangle, is used to cover

an angular sector of 120, forming what is usually referred to as a sect oral array.


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Figure: Examples of stationary, retractable/telescopic and embedded/hidden antennas used

in commercial cellular and cordless telephones, walkie-talkies, and CB radios.


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Figure 4.23 Triangular array of dipoles used as a sect oral base-station antenna for mobile

Communication

1. Set-top TV antennaThe most common dipole antenna is the type used with televisions, often colloquially referred to as "rabbit ears" or

"bunny ears". While in most applications the dipole elements are arranged along the same line, rabbit ears are

adjustable in length and angle. Larger dipoles are sometimes hung in a V shape with the center near the radio

equipment on the ground or the ends on the ground with the center supported. Shorter dipoles can be hung

vertically. Some have extra elements to get better reception such as loops (especially for UHF transmissions), which

can be turntable around a vertical axis, or a dial, which modifies the electrical properties of the antenna at each dial

position.


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Array antennaIntroduction

Several antennas can be arranged in space and interconnected to produce a

directional radiation pattern such a configuration of multiple radiating element is

referred to us an array antenna or a simple array. The introduction of shortwave

radio equipment in the 1920s made possible the use of reasonably sized antenna

arrays, thereby providing a convenient way to achieve a directive radiation pattern

for radio communications. During World War II UHF and microwave frequencies

and above are used extensively in satellite communication system.

2. Shortwave antenna

Dipoles for longer wavelengths are made from solid or stranded wire. Portable dipole antennas are

made from wire that can be rolled up when not in use. Ropes with weights on the ends can be the

connecting cable can be hoisted up with the ends on the ground or the ends hoisted up between two

supports in a V shape. While permanent antennas can be trimmed to the proper length, it is helpful if

portable antennas are adjustable to allow for local conditions when moved. One easy way is to fold the

ends of the elements to form loops and use adjustable clamps. The loops can then be used as

attachment points.

3. Military

US Military personnel occasionally use a 'doublet' antenna, especially during dismounted unconventiona

warfare. A radio operator may choose to bring several doublet antennas for different frequencies, such

as an antenna cut to length for the set MEDEVAC (medical evacuation) frequency, NCS (net control

station) frequency, and tactical frequency (the frequency used by troops in the field). This approach may

not be acceptable depending on the mission. Note that a doublet antenna will not work with thestandard SINCGARS radio when using frequency hopping(FH) but is effective for single channel (SC).

A doublet antenna is more practical for radios not intended for FH.


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Chapter 3

Arrays are found in many geometrical configurations. The most elementary is that of a linear array in

which the array element center lie along a straight line. The elements are may be equally or unequally

spaced. The radiation pattern of an array is determined by the type of individual element used, their

orientation, their positions in space, and the amplitude and phase of currents feeding them.

An array of antenna elements is a spatially extended collection of N similar radiators or elements, where

N is a countable number bigger than 1, and the term "similar radiators" means that all the elements have

the same polar radiation patterns, orientated in the same direction in 3-d space. The elements don't have to

be spaced on a regular grid, neither do they have to have the same terminal voltages, but it is assumed that

they are all fed with the same frequency and that one can define a fixed amplitude and phase angle for the

drive voltage of each element.

3.1 N-ELEMENT LINEAR ARRAY: UNIFORM AMPLITUDE AND SPACING

An array of identical elements all of identical magnitude and each with a progressive phase is referred to

as a uniform array. The array factor can be obtained by considering the elements to be point sources. If

the actual elements are not isotropic sources, the total field can be formed by multiplying the array factor

of the isotropic sources by the field of a single element. This is the pattern multiplication rule of (1), and it

applies only for arrays of identical elements.

E(total)=[E(single element at reference point)][array factor] (3-1)

The array factor is given by

(3-2)

Which can be written as


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(3-3)

Where

The array factor of (3) canalso be expressed in analternate, compact and closed form whose functions

and their distributions are more recognizable. This is accom-plished as follows.

(3-3a)

Which can be also written as

(3-3b)

If the reference point is the physical center of the array, the array factor of (5)

reduces to

(3-3c)

For small values of, the above expressioncanbe approximated b y


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(3-3d)

The maximum value of (6a) or (6b) is equal toN. To normalize the array factors so that the maximum

value of each is equal to unity, (6a) and (6b) are written in normalized form as

(3-3e)

And also writen as

(3-3f)

The maximum values of (3-3f) occur when

The array factor of (9) has only one maximum and occurs whenm=0 in(6-12). That is

(3-4)

which is the observation angle that makes =0.

The 3-dB point for the array factor of (9) occurs when


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(3-5)

For large values ofd(d), it reduces to

(3-5a)

The half-power beamwidth4h can be found once the angles of the first maximum(m) and the half-power

point (h) are determined. For a symmetrical pattern

(3-5b)

For the array factor of (9), there are secondary maxima (maxima of minor

lobes) which occur approximatelywhen the numerator of (9) attains its maximum

value. That is,

(3-6)

which canalso be written as

(3-7)

For large values ofd(d), it reduces to

(3-7a)


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The maximum of the first minor lobe of (8) occursapproximately when

(3-8)

Or when

At that point, the magnitude of (9) reduces to

Which in dB equal to

Thus the maximum of the first minor lobe of the array factor of (9) is 13.46 dB down from the maximum

at the major lobe. More accurate expressions for the angle, beamwidth, and magnitude of first minor lobe

of the array factor of (9) can be obtained.

3.2 N-ELEMENT LINEAR ARRAY: DIRECTIVITY

3.2.1 Broadside Array

As a result of the criteria for broadside radiationgivenby (6-18a), the array factor for this form of the array

reduces to

(3-9)


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which for a small spacing between the elements (d


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By making a change of variable, that is,

(22) can be written as

(3-13a)

For a large array (Nkd/2large), (25) can be approximated by extending the limits to infinity. That is,

(3-13b)

Since

(26)reduces to

(3-13c)

The directivity of (21) can be writen as

(3-14)

Using


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whereLis the overall length of the array, (28) can be expressed as

(3-14a)

Which for a large array (L>>d) reduce to

(3-14b)

3.2.2 Ordinary End-Fire Array

For anend-fire array, with the maximum radiationin the = direction, the array factor is givenby00

(3-15)

which, for a small spacing between the elements (d


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(3-16)

(3-17)

whose maximum value is unity (Umax=1) and it occurs = The average value of the radiation intensity 00

is given by

(3-16a)

By letting

(3-17a)

(3-16a) can be written as


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For a large array (Nkdlarge), (36) can be approximated by extending the limits to infinity. That is,

(3-18)

And the directivity to

(3-19)

which for a large array (L>>d) reduces to

Noted that the directivity of the end-fire array, is twice that for the broadside array .

3.2.4 Hansen-Woodyard End-Fire Array

For an end-fire array with improved directivity (Hansen-Woodyard designs) and max-imum radiationinthe

=0 direction, the radiation intensity (for small spacing between the elements,d


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The maximum radiation intensity is unity (Umax=1), and the average radiation intensity is given by

(3-20a)

where q and are defined, respectively, by

And

By taking the minimum value oof u

The radiation intensity reduces to

(3-20b)

which canalso be written as

(3-20c)

The average value of the radiation intensity is 0.554 times that

for the ordinary end-fire array . Thus the directivity can be expressed as:


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(3-21)

which is 1.805 times that of the ordinary end-fire array

(3-21a)

which for a large array (L>>d) reduces to

(3-21b)

TABLE of Directivities for Broadside and End-Fire Arrays


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Application of Array Antenna

An array antenna is used in various mobile communications systems, including land-mobile,

indoor-radio, and satellite-based system. An application of antenna arrays has been suggested in recent

years for mobile communications systems to over-come the problem of limited channel bandwidth,

thereby satisfying an ever growing demand for a large number of mobiles on communications channels.

When an array is appropriately used in a mobile communications system, it helps in improving the

system performance by increasing channel capacity and spectrum efficiency, extending range coverage,

tailoring beam shape, steering multiple beams to track many mobiles, and compensating aperture

distortion electronically. It also reduces multipath fading, co channel interferences, system complexity

and cost, BER, and outage probability.An array of antenna is used in a variety of ways to improve the performance of a communications

system. Perhaps most important is its -capability to cancel co-channel interferences. An array works on

the premise that the desired signal and unwanted co-channel interferences arrive from different

directions. Arrays are used in various configurations for mobile communications. The beam pattern of

the array is adjusted to suit the requirements by combining signals from different antennas with

appropriate weighting. Arrays antenna are used for the purpose of transmission and reception signal. An

important application of phased array antennas is that of adaptive arrays. This is where the control

signals used to create a particular radiation pattern are generated in a manner so as to optimize some

aspect of antenna performance such as its ability to automatically track one or more targets, to place

nulls strategically, or some other user-defined function. The advantages of an array of antenna in a

mobile communications system and improvements that are possible by using multiple rather than singl

antennas in a system.A Base StationThe system consists of a base station situated in a cell and serves a set of mobiles within the cell. It

transmits signals to each mobile and receives signals from them. It monitors their signal strength and

organizes the handoff when mobiles cross the cell boundary. It provides the link between the mobiles

within the cell and the rest of the network. The area served by a mobile phone system is divided in small


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areas known as cells. Each cell contains a base station, which is connected to a switching center,

communicates to mobile phones on the site by radio links, and connects these mobiles to the public

switching telephone network. A typical setup is shown in Fig. 1. Two types of radio channels are used:

control channels to carry control signals and traffic channels to carry. messages In mobile

communications literature, the transmission from a base station to a mobile is given many names, such as

downstream, forward link, and downlink. The corresponding terms for the transmission from a mobile

to a base are upstream, reverse link, and uplink. Multiple antennas at the base station may be used to

form multiple beams to cover the whole cell site.1. Call Initiation from a Mobile:

When a mobile phone is switched on, it scans the control channels and is tuned to the channel with the

strongest signal, usually arriving from the nearest base station. The phone user identifies itself and

establishes authorization to use the network. The base station then sends this message to the switching

center connected to the telephone network, which controls many Base stations. It assigns a radio traffic

channel to the phone under consideration, as the control channels are used by all phones in that area and

cannot be used for data traffic. Once the traffic channel is assigned, this information is relayed to the

phone via the base and the phone tunes itself to this channel. The switching center then completes the

rest of the call.2. Initiation of a Call to a Mobile:

When someone calls a mobile phone, the switching center sends a paging message through several base

stations. A phone tuned to a control channel detects its number and responds by sending a response

signal to the nearby base, which then informs the switching center about the location of the phone. The

switching center assigns a channel, and the call is completed.3. Registration:

A mobile is normally located by transmitting a paging message from various base stations. When a large

number of base stations are involved in the paging process, it becomes impractical and costly. This

problem is avoided by a registration procedure where a roaming phone registers with a base closer to

itself. This information may be stored with the switching center of the area as well as the home switching


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center of the phone. The home base of the phone is the one where it is permanently registered. Once a

call is received for this phone, its home switching center contacts the switching center where the phone

is currently roaming. Paging in the vicinity of the previous known location helps to locate the phone.

Fig. 1 A typical setup of a base mobile system.

Use of Arrays in Transmit Mode:-Antenna arrays have an equal role to play in both the receive and transmit

modes. The transmitting co-channel signals using an antenna array to

several mobiles is addressed in such that each mobile receives its desiredsignal with minimum cross talk with the other signals.Nulling Delayed Arrivals:-A frequency-hopping system also be used for correcting degradation due to fading. The system is useful in


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frequency-selective fading, where the different frequencies fade differently. Application of adaptive

arrays for frequency-hopping communications systems is described in whereas the use of adaptive arrays

for direct-sequence spread-spectrum communications system.


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single dipole and n-array antenna

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