chapter one introduction to antenna systems.pdf

8/22/2019 Chapter One Introduction to Antenna Systems.pdf

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Antennas and Radio Wave

Propagation

(Eeng 5153)

Introduction to AntennaSystems

By: Amare KassawBy: Amare KassawBy: Amare KassawBy: Amare Kassaw

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Lecture Outlines

Basic principles of antennas

Types of antennas

Electromagnetic radiations mechanisms

Radiation integrals and auxiliary potential functions

Far field radiation assumptions

Reciprocity theorems in antenna systems

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Basic Principles of Antennas

What is an antenna?

The American Heritage Dictionary: A metallic apparatus for sending and

receiving electromagnetic waves.

Websters Dictionary: Usually a metallic device (as a rod or wire) for

.

Balanis, Antenna Theory: An antenna is a passive transitional structure

between free-space and a guiding structure that radiate EMW

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Source

Tx

Receiver

Circuit

Rx

Free Space


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Definition Continued

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The guiding device or transmission line may be:

Coaxial transmission line

Hollow pipe (wave guide)

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The antenna transmitting and receiving equivalent modes are

given as follows

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In addition to receiving & transmitting energy, an antenna in an

advanced wireless system is usually required to optimize or

accentuate the radiation energy in some directions and

suppress it in others.

Thus the antenna can also serve as a directional device in

addition to a probing device.

It must then take various forms to meet the particular need at

hand, and it may be a piece ofconducting wire, an aperture, apatch, an assembly of elements (array), a reflector, a lens, and

so forth (see the next section for detail)

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For wireless communication systems, the antenna is one of the

most critical components.

A good design of the antenna can relax system requirements and

improve overall system performance.

For example in TV system the overall broadcast reception can be

mprove y u z ng a g -per ormance an enna.

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Transmission line Thevenin equivalent of the antenna

system

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From the figure:

The source is represented by an ideal generator,

The transmission line is represented by a line with characteristic

impedance Zc

The antenna is represented by a load ZA connected to the

transmission lineZA = (RL + Rr ) + jXA

The load resistance RL is used to represent the conduction and

dielectric losses associated with the antenna structure

While Rris the radiation resistance that is used to represent

radiation by the antenna.10


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The reactance XA is used to represent the imaginary part of the

impedance associated with radiation by the antenna.

In ideal conditions, energy generated by the source should be

totally transferred to the radiation resistance Rr .

However, in a practical system there are conduction-dielectric

osses ue o e ossy na ure o e ransm ss on ne an e

antenna, as well as those due to reflections (mismatch) losses at the

interface between the line and the antenna.

Taking into account the internal impedance of the source and

neglecting line and reflection (mismatch) losses, maximum power

is transferred to the antenna under conjugate matching conditions.11


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The reflected waves from the interface (along with the travelling

waves from the source) create constructive and destructive

interference patterns (Standing Waves) inside the transmission line

which represent pockets of energy concentrations and storage.

If the antenna system is not properly designed, the transmission line

cou ac as an energy s orage e emen ns ea o a wave gu ng an

energy transporting device.

If the maximum field intensities of the standing wave are sufficiently

large, they can cause arching inside the transmission lines.

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How to minimize the losses due to the line and antenna, and the

standing waves ?

Losses due to the line can be minimized by using low-loss lines

While loses duet to the antenna can be decreased by reducing the

loss resistance represented byRL .

The standing waves can be reduced and then the energy storagecapacity of the line can be minimized by matching the

impedance of the antenna (load) to the characteristic impedance of

the line.

This is the same as matching loads to transmission lines , where the

load here is the antenna, 13


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Exercise/Assignment

Explain the basic differences between

Transmission lines/waveguides

Radiators and

Resonators

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Types of Antenna

Antennas can be classified based on EM radiation,

size/dimension, and type /application

Classification based on EM radiation

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Classification based on size/dimension of an antenna

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Classification based on type of an antenna

1. Wire Antenna

Wire antennas are seen everywhere on automobiles, buildings,

ships, aircraft, spacecraft, and so on.

There are various shapes of wire antennas such as a straight wire

, , .

Loop may be circular, rectangular , square, ellipse, or any other

configuration.

The circular loop is the most common because of its simplicity in

construction.

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Figure : Wire Antenna Configurations


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2. Aperture Antennas

It contains some sort of opening through which electromagnetic

waves are transmitted or received.

It has better utilization of higher frequencies.

Hence it is most common at microwave frequencies.

ntennas of this type are very useful foraircraft andspacecraft applications,because they can be very conveniently

flush-mounted on the skin of the aircraft or spacecraft.

In addition, they can be covered with a dielectric material to

protect them from hazardous conditions of the environment.

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Figure : Aperture Antenna Configurations


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3. Microstrip Antennas

The metallic patch can take many different configurations.

However, the rectangular and circular types are the most popular

because of ease of analysis and fabrication, and their attractive

radiation characteristics.

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Figure : Microstrip Antennas


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The microstrip antennas:

Has low profile, conformable to planar and nonplanar surfaces,

Simple and inexpensive to fabricate using modern printed-circuit

technology, mechanically robust when mounted on rigid surfaces

Compatible with MMIC designs and

Very versatile in terms of resonant frequency, polarization,

pattern, and impedance.

This antenna can be mounted on the surface of high performance

aircraft, spacecraft, satellites, missiles, cars, and even handheld

mobile telephones.

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4. Array Antennas

Many applications require radiation characteristics that may not be

achievable by a single element.

It may, however, be possible that an aggregate of radiating elements

in an electrical and geometrical arrangement (an array) will result in

the desiredradiation characteristics.

The arrangement of the array may be such that the radiation from the

elements adds up to give a radiation maximum in a particular

direction and minimum in others as desired.

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5. Reflector Antenna

Because of the need to communicate over great distances,

sophisticated forms of antennas had to be used in order to transmit

and receive signals that had to travel millions of miles.

A very common antenna form for such an application is a parabolic

.

Antennas of this type have been built with diameters as large as

305 m. Such large dimensions are needed to achieve the high gain

required to transmit or receive signals after millions of miles of

travel.

Another form of a reflector is the corner reflector. 30


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Figure : Reflector Antennas


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Figure: The big dish antenna of Stanford University.


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6. Lens Antennas

Lenses are primarily used to collimate incident divergent energy to

prevent it from spreading in undesired directions.

By properly shaping the geometrical configuration of the lenses, theycan transform various forms of divergent energy into plane waves.

ey can e use n mos o e same app ca ons as are e para o c

reflectors, especially at higher frequencies. Their dimensions and

weight become exceedingly large at lower frequencies.

Lens antennas are classified according to the material from which

they are constructed, or according to their geometrical shape.

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Electromagnetic Wave Radiation Mechanisms

How is radiation achieved by the antenna?

How EMF(E,H) is :

Generated by the source

Contained and guided within the transmission line and the antenna

Electric charges/disturbances are required to excite the fields.

Once the disturbance in the EMW has been initiated, EMWs are

created which begin to travel outwardly.

The waves do not stop or extinguish themselves even if the disturbance

has been removed.

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The created electromagnetic waves travel inside the transmission

line, then into the antenna, and finally radiated as free-space

waves, even if the electric source has ceased to exist.

If the electric disturbance is of a continuous nature,

electromagnetic waves exist continuously and follow in their

travel behind the others.

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When the electromagnetic waves are within the transmission line and

antenna, their existence is associated with the presence of the

charges inside the conductors.

However, when the waves are radiated, they form closed loops and

no need of charges to sustain their existence.

Hence electric charges are required to excite the fields but are

not needed to sustain them and may exist in their absence.

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Basic Sources of Radiation

1. EM Radiation from Single Wire

Conducting wires are material which is used to allow the motion

of electric charges and the creation of current flow.

Let us consider a very thin wire shown in figure below

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For the wire is very thin , the current in the wire is given by

If the current is time varying, then the derivative of the current is

Then for a wire of lengthl , the equation becomes

This equation is the basic relation between current and charge, and

it is the fundamental relation ofelectromagnetic radiation .

It simply states that to create radiation, there must be a time-varying

current or an acceleration (or deceleration) of charges.

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Hence based on the state of the charges, we concluded that :

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Wire configuration to create radiation

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2. EM Radiation from Two Wires:

E-field Analysis

Applying a voltage across the two-conductor transmission line

creates an electric field between the conductors.

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Associated with the electric field has electric lines of force which

are tangent to the electric field at each point and their strength is

proportional to the electric field intensity.

The electric lines of force have a tendency to act on the free

electrons associated with each conductor and force them to be

displaced.

The electric field lines drawn between the two conductors help to

exhibit the distribution of charge.

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If we assume that the voltage source is sinusoidal, the electric field

between the conductors will also be sinusoidal with a period equal

to that of the applied source.

The relative magnitude of the electric field intensity is indicated by

the density (bunching) of the lines of force with the arrows showing

the relative direction (positive or negative).

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H-filed Analysis

The movement of the charges creates a current that in turn creates

a magnetic field intensity.

Associated with the magnetic field intensity are magnetic lines of

force which are tangent to the magnetic field.

The creation of time-varying electric and magnetic fields between

the conductors forms electromagnetic waves which travel along

the transmission line (see next figure).

The electromagnetic waves enter the antenna and have associated

with them electric charges and corresponding currents.

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Figure : Source, transmission line, antenna, and detachment of

electric field lines.


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The continuous nature of theelectric disturbance, creates a

continuous type EMW which

follow in their travel one after

the other.

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Conclusion :

When the electromagnetic waves are within the transmission line and antenna, their

existence is associated with the presence of the charges inside the conductors.

However, when the waves are radiated, they form closed loops and there are nocharges to sustain their existence.

Hence electric charges are required to excite the fields but are not necessary to

sustain them and may exist in their absence.


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Figure a shows the lines of force create between the arms of a small center-fed dipole


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in the first quarter of the period during which time the charge has reached its

maximum value and the lines have travelled outwardly a radial distance/4.

During the next quarter of the period, the original three lines travel an additional/4

(a total of /2 from the initial point) and the charge density on the conductors begins

to diminish.

This can be accomplished by introducing opposite charges which at the end of the

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rst a o t e per o ave neutra ze t e c arges on t e con uctors.

The end result is that there are three lines of force pointed upward in the first/4

distance and the same number of lines directed downward in the second/4. Since

there is no net charge on the antenna, then the lines of force must have been forced to

detach themselves from the conductors and to unite together to form closed loops. In

the remaining second half of the period, the same procedure is followed but in the

opposite direction.

Radiation Integrals and Auxiliary Potential Functions


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Radiation Integrals and Auxiliary Potential Functions

Introduction EMW problems may be synthesis or analysis type.

EMW synthesis problems :

First specify the EM radiated fields(E &H) and

Then we determine the EM source (J & M)of the radiated fields

EMW analysis problems:

Then we determine the EM radiation(E&H) by the sourcesHow to solve analysis problems ?

Direct integration from the sources.

By using auxiliary functions (vector potentials) such as The magnetic and electric vector potential ( A & F

respectively) or

Equivalently the Hertz potential ( e and h)54


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The vector potentials are strictly a mathematical tool used to

determine thephysical quantities E & H.

They are used to determine an easy way to determine the radiated

fields as shown below.

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Although both path requires integration, the integration over path two


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Although both path requires integration, the integration overpath two

is much simpler. The most difficult operation in the two-step procedure is the

integration to determine A and F.

Once the vector potentials are known, then E and H can always be

determined because any well behaved function, no matter how

complex, can always be differentiated.

The integration required to determine the potential functions is

restricted over the bounds of the sources J and M. This will result in

the A and F to be functions of the observation point coordinates.

So, the differentiation to determine E and H must be done in terms of

the observation point coordinates.

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Derivation of electromagnetic fields (E & H) from the


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Derivation of electromagnetic fields (E & H) from the

sources(J &M) via the two step procedure

To determine the radiated fields, we use the basic vector identities,

Maxwells and EMW equations ( Refer the proof from EMW and Guided( Refer the proof from EMW and Guided( Refer the proof from EMW and Guided( Refer the proof from EMW and GuidedStructures)Structures)Structures)Structures)

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Where A is vector Vis scalar

Maxwells Equations


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EMW Equations

Divergence and curl of potential functions

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Steps to derive electric and magnetic fields


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p g

First, find the auxiliary vector potential functions A and F

generated, respectively, by J and M.

Determine the corresponding electric and magnetic fields

(EA,HA due to A and EF ,HF due to F).

The total fields are then obtained by the superposition of theindividual fields due to A and F (J and M).

In summary, the steps that can be used to find the fields areas follows:

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1. Specify the sources J and M


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2. Find A due to J by using

and F due to M by using

3. Determine H and E due to the magnetic vector potential A by

and determine H and E due to the electric vector potential F by

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4. Finally, the total radiated fields are give by


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Far Field Radiation Assumptions


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Use the hand out

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Duality Theorems in Antenna System


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Use the hand out

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Reciprocity Theorems in Antenna System


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Use the hand out

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chapter one introduction to antenna systems.pdf

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