antenna

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© Prepared by Mr. V. D. Gaikwad, Vishwakarma Institute of Technology, Pune.

Antennas and Wave Propagation

---- V. D. GaikwadV. D. GaikwadV. D. GaikwadV. D. Gaikwad

Unit : 5

V. D. Gaikwad, Vishwakarma Institute of Technology, Pune

Antenna : Definition

� “Antenna is one or more electrical conductors of a

specific length that radiate radio waves generated by a

transmitter or that collects radio waves at the receiver.”

� Radio wave is an electromagnetic wave.

� E and H fields are perpendicular to one another.

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Electromagnetic Wave


EM wave traveling !!!

Animation

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Polarization

� The direction of electric field specifies the polarization of the

antenna.

� Horizontally polarized wave = Electric field is parallel to the

earth

� Vertically polarized wave = Electric field is perpendicular to the

earth


Horizontally Polarized Antenna

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Horizontally Polarized Antenna


Vertically Polarized Antenna

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Polarization contd …

� Transmitting and receiving antennas must be of the same

polarization for optimum transmission and reception.

� Vertically polarized wave will produce 0 V in a horizontal

antenna and vice-a versa.


Dipole Antenna

� Half-wave dipole

- also called doublet

- it is a piece of wire or rod that is one

half wavelength long at the operating

frequency

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A Half-Wave Dipole Antenna

λλλλ / 2

transmission line

73 ohm impedance

75 ohm coax


Folded Dipole

λλλλ / 2

transmission line

300 ohm impedance

300 ohm Twin lead

Commonly used for TV and FM radio reception

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Folded Dipole


Radiation Pattern

� “A diagram which indicates how well an antenna transmits or

receives in the different directions is called radiation pattern of

the antenna.”

� Isotropic antenna – Omni directional pattern

� Yagi antenna – Unidirectional pattern

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Uni - Directional Pattern


Bi-Directional Pattern

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Omni Directional Pattern


Dipole Antenna

++++ ++++ ++++ ++++

−−−− −−−− −−−− −−−−

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++++

++++

−−−−

−−−−

Conductor bending


++++

−−−−

Conductor bending at 900

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Animation 1

How waves are traveling into the space !!!

Animation 2


Yagi Antenna

� Most common antenna for TV reception

� Gain = 7 dB

� Radiation pattern is UNI-DIRECTIONAL

� Three elements

1. Reflector

2. Director

3. Dipole

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Yagi Antenna


Design Issues

� Reflector

- 5% longer than dipole

- distance between dipole and reflector is 0.2 λλλλ

� Director

- 5% shorter than dipole

- distance between dipole and director is 0.15 λλλλ

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Design Issues …

� Length of Reflector (m) = 152

f (MHz)

� Length of Dipole (m) = 143

f (MHz)

� Length of director = 137

f (MHz)


� e.g. For 143 MHz signal,

� Reflector Length = 1.06 m

� Dipole Length = 1.00 m

� Director Length = 0.96 m

Design Issues …

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1. Antenna Gain

2. Field Intensity

3. Antenna Resistance

4. Bandwidth

5. Beamwidth

Antenna Parameters


Directive Gain

� “Ratio of the power density in a particular direction of one

antenna to the power density that would be radiated by an

omni-directional antenna (isotropic antenna)”

1. Antenna Gain

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� “The field intensity (field strength) of an antenna’s radiation, at

a given point in space, is equal to the amount of voltage

induced in a wire antenna 1 m long, located at that given

point.”

� Affected by number of conditions such as the time of day,

atmospheric conditions and distance.

2. Field Intensity


� “Radiation resistance is the ratio of the power radiated by theantenna to the square of the current at the feed points.”

� Rrad =

Prad

I2

3. Radiation Resistance

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� “The range of the frequencies over which the antenna will

radiate effectively”

� The antenna will perform satisfactorily throughout this range of

frequencies.

4. Bandwidth


� “Beamwidth is defined as angles created by comparing the half-

power points (3dB) on the main radiation lobe to its maximum

power point.”

5. Beamwidth

antenna

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