Lect Antenna Chap 01 Part 1

8/2/2019 Lect Antenna Chap 01 Part 1

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EEE-463 Antenna and Radio Wave Propagation


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What is Antenna?

In any wireless communication system, there is one

component which fundamentally affects the

performance of the system and if it is not working

properly, then the system as a whole will not. Thiscomponent is the antenna

A transitional structure between free-space and a

guiding device

The primary function of an antenna is to couple RFpower into free space at the transmitter end and

retrieve it again at the receiver end losing as little

power and information as possible


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WHAT IS PROPAGATION?

Early man and a need to communicate beyond the range

of the human voice

Development of alternate methods of communication,

but limited in range Development of courier and postal systems but still

speed constraints

For centuries the time required for the delivery of a

message depended on the speed of a horse

The invention of the telegraph

Then a short time later, man discovered how to transmit

messages in the form of RADIO WAVES


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How waves travel through air.

The object producing the waves is called the SOURCEa

bell in this illustration

The object responding to the waves is called a

DETECTOR or RECEIVERin this case, the human ear The medium is air, which is the means of conveying the

waves from the source to the detector

As the waves are produced, they carry energy through

the medium (air) to the detector (ear)


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The electromagnetic energy of light is a form of

electromagnetic radiation. Light and similar forms of

radiation are made up of moving electric and magnetic

forces and move as waves

Electromagnetic waves move in a manner similar to the

waves produced by the pebble dropped in the pool of

water


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Simple radio communication

system

Transmitter: an electronic device that generates radio-

frequency energy

Antenna: converts the energy into radio waves that

radiate into space from the antenna at the light speed

If another antenna is placed in the path of the radio

waves, it absorbs part of the waves and converts them

to energy


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Radio Frequency

Radio frequency (RF) is a rate of oscillation in the range

of about 3 kHz to 300 GHz, which corresponds to

the frequency of radio waves, and the alternating

currents which carry radio signals. Electric currents that oscillate at radio frequencies have

special properties such as:

The energy in an RF current can radiate off a

conductor into space as electromagnetic waves

Skin effect of RF current


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Transmission line may take the form of coaxial cable or

any other form, for the transportation of

electromagnetic waves


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Equivalent of Antenna System

The antenna is represented by a load ZA

[ZA = (RL + Rr ) + jXA]

The load resistance RL is used to represent theconduction and dielectric losses associated with the

antenna structure Rr referred to as the radiation resistance, is used to

represent radiation by the antenna

The reactance XA is used to represent the imaginary

part of the impedance associated with radiation by theantenna

Ideal conditions and practical system (conduction-dielectric losses, mismatch losses)


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Introduction

The reflected waves from the interface create, along

with the travelling waves from the source toward the

antenna, constructive and destructive interference

patterns, referred to as standing waves, inside thetransmission line

Standing waves represent pockets of energy

concentrations and storage


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Introduction...

If the antenna is not properly designed than

transmission line could act as a energy storage

element instead of wave guiding device

The standing waves can be reduced, and the energystorage capacity of the line minimized, by matching the

impedance of the antenna (load) to the characteristic

impedance of the line

The antenna must also serve as a directional device inaddition to receiving or transmitting energy


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Types of Antennas

There are 7 main types of antennas, the grouping of

which is based primarily on the way they operate,

rather than the use to which they are put. The groups

are as follows:-


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Element/Wire antennas

Element antennas consist of simple structure based on

wires or rods (ie: the elements). A number of examples

are shown below (L-R), dipole, monopole, loop, helix


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Element antennas

The definition characteristic of element antennas is

that they are almost always resonant structures (An

antenna is said to be resonant if its input impedance is

entirely real, i.e. Zin = R + j*0), and hence havedimensions which are commensurate with the signal

wavelength

These antennas have a wide range of general

characteristics with regard to gain, polarization etc. Current distribution, fields radiated in the far field,

Directivity and other properties will be discussed later


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Aperture antenna

The type of antenna is exclusively used with rectangular orcircular waveguide, since they are formed in most cases,

by flaring out the open end of a waveguide

Frequency range is restricted to UHF/microwave region

The reason they are called aperture antennas is because

the collecting area of the antenna is precisely defined by

the physical size of the hole. However, the aperture of

antenna is a term which is applied to all antennas

High gain

wide impedance bandwidth

Aperture/horn antenna


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Printed Antennas

Printed patch radiators come in all shapes, sizes andconfiguration. However the basic patch antenna

consists of a metallic patch on a dielectric substrate

whose shape is formed by the etching process

Frequency range for this type of antenna are usually

GHz region but having small bandwidth.

They are low profile, inexpensive to fabricate using

modern printed-circuit technology They can be mounted on the surface of high-

performance aircraft, spacecraft, mobile telephones

etc


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Array Antennas

The aggregate of radiating elements

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 or directions,minimum in others, or otherwise as desired

- Yagi Uda Array

- Aperture Array

- Microstrip Patch Array

- Slotted-waveguide array


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Most brilliant antenna designs, simple to construct andhas a high gain, typically operate in the HF to UHF

bands (about 3 MHz to 3 GHz), although

their bandwidth is typically small

Yagi antenna consists of a single 'feed' or 'driven'

element, typically a dipole

Rest of the elements are parasitic - they reflect or help

to transmit the energy in a particular direction

Yagi Uda Array


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Reflector Antennas

Reflector antennas are utilize the fact that

electromagnetic waves can be reflected to a focal point

using a shaped metal plate

It should be pointed out that thereflector itself is not that antenna and

therefore some form of conventional

antenna is placed at the focal point of

the reflector

The most widespread application is

of course the satellite receiving antenna


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Leaky Wave antennas

These antennas formed from dielectric mm-

waveguides and are predominantly used above 30 GHz

and up into the infrared region. They make use of the

fact that radiation occurs from discontinuities intransmission lines. An example of such antenna is

shown below


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Lens antennas

Just as light can be refracted and collimated by a glasslens, so can microwaves, though it is usually only haspractical applications above 10GHz, an example of thisis poly-rod feeds which have been

used on satellite dishes instead

of horn feeds, at 12GHz

The figure below shows a

microwave prism made using afoam mould containing styrene

pellets

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Lect Antenna Chap 01 Part 1

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