lect antenna chap 01 part 1
TRANSCRIPT
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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