Transcript Wireless Communications and Networks

Antennas
Lecture 9
Introduction
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An antenna is an electrical conductor or
system of conductors. An antenna is a
device whose function is to radiate
electromagnetic energy or to intercept
electromagnetic radiation
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Transmission - radiates electromagnetic energy
into space
Reception - collects electromagnetic energy
from space
Introduction
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A transmitting antenna can also be used for
reception and vice versa. In two-way
communication, the same antenna can be
used for transmission and reception.
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This property of interchangeability is
known as antenna reciprocity.
Introduction
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The received signal strength is described in
terms of electric field strength. If a signal
induces a 15 mV in a receiving antenna 3m
long, then the field strength is 15 mV/3m or 5
mV/m.
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For maximum power transfer the antenna
must be able to match the transmission and
the load in terms of impedance.
The polarization must be the same in the receive and
transmit antennas. The polarization is the direction of
the electric filed.
Introduction
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At radio frequencies a wire can serve as an
impedance matching device.
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The spacing, length and shape of the device
are related to wavelength of the transmitter.
Introduction
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When RF energy is fed into a transmission
line standing waves occur.
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Energy is lost or radiated into the space
surrounding the line.
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By separating the ends of the transmission
line, a greater surface area is exposed and
this enhances the radiation process.
Introduction
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Greater efficiency is achieved if the two lines
are at right angles to each other.
The electric and magnetic fields are now fully
coupled into the surrounding space instead of
being confined between the wires. This type
of radiator is called a dipole.
Radiation Patterns
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Graphical representation of radiation properties of an
antenna. A radiation pattern is a polar diagram showing
field strengths or the power densities at various angular
positions relative to the antenna.
Depicted as two-dimensional cross section
The distance from the location of the antenna to a point on
the radiation pattern indicates the strength of the radiation
in that direction.
This means that antennas do not perform equally well in
all directions
Length Calculations
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The radiation pattern depends mainly on the length of the
antenna. The length of an antenna can be calculated using the
following equation, the velocity factor of wire is 95% compare
to air.
c
L k
f
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where c is the speed of light
L is the length in meters
f is frequency in Hertz
and k is the velocity factor
Length Calculations
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Example
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It is required that an antenna of the half-dipole
type be built to receive broadcast at 100 MHz,
calculate the optimum length of the antenna.
Beam width
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It is sometimes necessary to compare the directivity of
antennas very quickly, the beam width provides such a
quick comparison.
The beam width of an antenna is the angle within which
the power radiated is above one half of what it is in the
most preferential direction, or the angle when the voltage
remains within 70.7% of the voltage developed when the
antenna is aimed at the most preferential direction.Beam
width (or half-power beam width)
Measure of directivity of antenna
Antenna resistance
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For power to get to an antenna it must be connected to a
transmission line.
To prevent standing waves from occurring within the line and
for maximum power transfer, the resistance of the transmission
line must be equal to the resistance of the antenna.
The antenna resistance is termed radiation resistance. This is
defined as a fictitious resistance which would dissipate as much
power as an antenna in question is radiating if it were connected
to the same transmission line.
If an antenna is radiating 100 W when drawing a current of 2 A
then its radiation resistance will be 25 ohm. (P=I2R).
Antenna resistance
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Not all energy absorbed by an antenna is radiated.
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Losses can occur within the antenna (imperfect dielectrics, eddy
currents etc), as such antenna efficiency is defined
Ptransmitted
Rr


Pinput
Rr  Rl
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Rr is the resistance of the antenna
Rl is resistance due to losses
Types of Antennas
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Isotropic antenna (idealized)
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Dipole antennas
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Radiates power equally in all directions
Half-wave dipole antenna (or Hertz antenna)
Quarter-wave vertical antenna (or Marconi
antenna)
Parabolic Reflective Antenna
Half-wave dipole antenna (or Hertz antenna)
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When the total length of the two wires is
half the wavelength, the antenna is called a
half wave dipole. This is also known as the
Hertz antenna.
Quarter-wave vertical antenna (or Marconi antenna)
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In this case, the total length of the two wires is a
quarter of the wavelength.
This type of antenna is also referred to as the
Marconi antenna.
It is normally used at frequencies below 2 MHz.
This antenna requires a conducting path to ground.
The ground is thus used as the other quarter
wavelength. Electrically therefore it acts as a half
wavelength.
Antenna Gain
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Antenna gain
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Power output, in a particular direction,
compared to that produced in any direction by
a perfect omnidirectional antenna (isotropic
antenna)
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If an antenna is said to have a gain of 10dB, it means it improves
upon the reference antenna in that direction by 10dB.
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The increased power in one direction is at the expense of
other directions.
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Effective area
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Related to physical size and shape of antenna
The increased power in one direction is at the expense of
other directions. This gain is a power ratio and can be
expressed as:
P2
G (dB)  10 log
P1
The actual amount of power received by an antenna through
free space can be predicted by use of the following equation
Pt Gt Gr 
Pr 
2 2
16 d
2
Antenna Gain
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Relationship between antenna gain and effective
area
G
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4Ae
2
4f Ae

c2
2
G = antenna gain
Ae = effective area
f = carrier frequency
c = speed of light (» 3 ´ 108 m/s)
 = carrier wavelength
Example
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Two half-dipole antennas each with a
gain of 1.64 are separated by a distance
50 km. The transmitter feeds its
antenna with 10 W at 144 MHz.
Calculate the power received.
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A half wave dipole antenna is capable of
radiating 1 kW and has a gain of 2.15 dB
over an isotropic antenna. How much power
must be delivered to the isotropic antenna
match the field strength of the directional
antenna?
Reflectors and Directors
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It is sometimes necessary to focus
power in one particular direction. This
can be done by the use of reflectors
and directors.