01 - Antenna Parameters[1]

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

MALAYAN COLLEGE LAGUNA - MITL

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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_.pdf


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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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01 - Antenna Parameters[1]