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8/2/2019 01 - Antenna Parameters[1]
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ECE143 TRANSMISSION MEDIA AND
ANTENNA SYSTEMS
Engr. Maria Cristina Feliz L. Odeste
ECE InstructorMalayan Colleges Laguna
3rd Term
SY 2011 - 2012
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ANTENNA PARAMETERS
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Introduction
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It is apparent that the size of an antenna is inversely
proportional to frequency. Every antenna has directional
characteristics and radiate more energy in certain directions
relative to other directions.
The radiation efficiency (ratio of radiated to reflected
energy) of an open transmission line is extremely low. To
radiate more energy, simply spread the conductors farther
apart. Such antenna is called a dipole.
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Radiation from a Transmission Line
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Introduction
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A basic antenna is a passive reciprocal device because it
cannot actually amplify a signal.
Transmit antennas must be capable of handling high power
and therefore must be constructed with materials that can
withstand high voltages and currents. Receive antennas
however, produce very small voltages and currents and can be
constructed from small-diameter wire.
A special coupling device called a duplexer can be used to
direct the transmit and receive signals and provide thenecessary isolation.
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Antenna Parameters
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Radiation Pattern is a polar diagram or graph representing
field strengths or power densities at various angular positions
relative to an antenna.
If the radiation pattern is plotted in terms of electric field strength or
power density, it is called Absolute Radiation Pattern
If it plots field strength or power density with respect to the value at
a reference point, it is called Relative Radiation Pattern.
The line bisecting the major lobe, or pointing from the center of the
antenna in the direction of maximum radiation is called the Line ofShoot.
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Radiation Patterns
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The radiation patters
shown are 2D, however
radiation from an
actual antenna is 3D.
Therefore radiation
patterns are taken in
both the horizontal andvertical planes.
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Unidirectional 3D Radiation Pattern
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Bidirectional 3D Radiation Pattern
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Antenna Parameters
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Radiation Field
Near Field refer to the field pattern that is close to the antenna.
The near field is sometimes called the Induction Field.
Far Field refers to the field pattern that is of great distance. Power
that reaches the far field continues to radiate outward and is neverreturned to the antenna.
The far field is sometimes called the Radiation Field.
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Antenna Parameters
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Not all the power supplied to an antenna is radiated, some of
it is converted to heat and dissipated.
Radiation Resistance is an AC antenna resistance and is
equal to the ratio of the power radiated by the antenna to the
square of the current at its feedpoint.
Radiation resistance is the resistance that, if it replaced theantenna, would dissipate exactly the same amount of power
that the antenna radiates.
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Equivalent Circuit of an Antenna
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Antenna Parameters
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Antenna Efficiency is the ratio of the power radiated by an
antenna to the sum of the power radiated and the power
dissipated.
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Antenna Parameters
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Antenna Gain
Directive Gain is the ratio of the power density radiated in a particular
direction to the power density radiated to the same point by a
reference antenna.
Power Gain is the same as directive gain exept that the total power fed
to the antenna is used (i.e., antenna efficiency is taken into account)
If an antenna is lossless, it radiates 100% of the input power, and the
power gain is equal to the directive gain.
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Antenna Parameters
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Effective Isotrpic Radiated Power (EIRP) is defined as the
equivalent transmit power and is expressed mathematically as
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Sample Problem
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For a transmit antenna with Rr = 72, Re = 8, directive gain
of 20 and Pin = 100W, determine:
Antenna efficiency
Antenna gain (absolute and dB)
Radiated power in watts, dB and dBm
EIRP in watts, dB and dBm.
90%
18/12.55 dB 90W/49.54 dBm/19.54 dBW
1800W/62.55dBm/32.55 dBW
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Antenna Parameters
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Antenna Capture Area and Captured Power
The Antenna Capture Area is the effective area and a parameter for
describing the reception properties of an antenna.
Captured Power is the power available at the antennas output
terminals (in watts) Captured Power is directly proportional to the received power density
and the effective capture area of the antenna. The captured power can
be delivered to a load such as a transmission line or a receivers input
circuitry.
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Antenna Directive Gain
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ANTENNA TYPE UNITLESS GAIN
ISOTROPIC 1
HERTZIAN 1.5
HERTZ 1.64
TURNSTILE 1.15
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Sample Problems
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Calculate the captured power 10km away from a half-wave
dipole transmitter with a 10W transmit power for the following
receive antennas at 150 MHz.
Hertzian dipole
Half-wave dipole
Answer: 6.23 nW, 6.81 nW
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Antenna Parameters
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Antenna Polarization refers simply to the orientation of the
electric field radiated from it.
Linear Polarization
Horizontally Polarized
Vertically Polarized
Elliptical Polarization
Circular Polarization
Right-Hand Circular Polarization
Left-Hand Circular Polarization
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Antenna Parameters
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Antenna Bandwidth is vaguely defined as the frequency
range over which antenna operation is satisfactory.
Bandwidth is normally taken as the difference between the
half-power frequencies and is often expressed as a
percentage of the antennas optimum frequency of operation.
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Sample Problem
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Determine the percent bandwidth for an antenna with an
optimum frequency of operation of 400 MHz and -3dB
frequencies of 380 MHz and 420 MHz.
Answer: 10%
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Antenna Parameters
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Antenna Beamwidth is simply the angular separation
between the two half-power points on the major lobe of the
antennas plane radiation pattern.
Antenna Beamwidth is sometimes called -3dB beamwidth or
half-power beamwidth.
Antenna Gain is inversely proportional to beamwidth.
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Beamwidth
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Specification Sheet
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http://../CTSDG-06515-6DM_.pdf8/2/2019 01 - Antenna Parameters[1]
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ACTIVITY
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How much power is dissipated by an antenna whose radiation
resistance is 100 and ohmic resistance of 10 ? The transmitter
power that supplied the antenna is 200W.
Answer:
A transmitter is required to deliver 100 W to an antenna through a45 m coaxial cable with a loss of 4 dB/100 m. What must be the
output power of the transmitter, assuming the line is matched?
Answer: 151 W
Determine the effective radiated power in dBm for an antenna withdirectivity of 300, efficiency of 80% and an input power of
2500W.
Answer: 87.78 dBm
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PROBLEM SET
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