Practical Antenna

8/11/2019 Practical Antenna

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Practical Antennaand

Methods of Excitations

Practical consideration

relating to feeding and

matching of antennas


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Effect of earth on vertical pattern

When antenna is isolated and close to earth, earth

has considerable effect on pattern.

This is significant due to the flow of current in earth

which modifies the pattern. Magnitude and phase of this current depend on,

frequency, conductivity and permeability of the

earth.

Earth is perfect conductor at low and medium

frequencies.


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Vertical and Horizontal antenna

Direct and reflectedrays changes theimage charge andcurrents are shown.

The actual andimage antenna havecharges of oppositepolarity. In case ofvertical antenna,

currents are in samedirection. But inhorizontal antennacurrents are inopposite direction.

Fig. Actual and image antenna


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Vertical Antenna

Fig. Vertical pattern of vertical antenna above the earth

When vertical antenna is above the earth then if antenna

height is h from the earth, its image height will be 2h.

Resultant field pattern is product of the antenna and the

vertical pattern of two uniformly excited non directionalradiators spaced at h.


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Horizontal Antenna

For horizontal antenna, vertical pattern is product of

vertical pattern of antenna and vertical pattern of

uniformly excited non directional radiator spaced at

lemda.

Radiation pattern of horizontal antenna above the earth


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Methods of excitation of antenna

Main four type methods for excitation:

a) Center-fed

b) Shunt-fed/ Delta matched excitation

c) End-fed

d) Tapped-fed

Good Impedance

matched

Standing wave can

be eliminated

Simplest method

Improper

impedance matching

High VSWR

Lower impedance

point

Reduces radiation

by wires

Standing wave

reduce as

mismatch reduced

Dipole is fed at

center by open-wire

transmission line


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Impedance Matching Techniques

Impedance matching can be done by using

1. Stubs

2. Folded dipole

3. Baluns


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Stubs

A stubs is a piece of transmission line one end of which is connected to

the transmission line for matching with the load impedance. Its purpose is to be tuned out the reactance.

Second end of stubs can be open circuited or short circuited.

Short circuited lines are used to avoid radiation losses.

Stub matching of transmission lines


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Baluns

The word derived from the BALanced to UNbalanced.

A dipole is a balanced load. In a dipole two arms haveequal current. The impedance of the arms to groundshould be same.

A balun is a balanced to unbalanced transformer.

It is a circuit element connected between a balanced lineand unbalanced line or an antenna.

As a coaxial line is unbalanced balun is connected betweenthe coaxial line and dipole.

Transmission Lines are used as a baluns at high frequenciesand center tapped transformer is used at lowerfrequencies.The primary is unbalanced. Center tappedsecondary winding is balanced and is connected to anantenna.


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Baluns are of two types.1. Broad band :Example: Conventional Transformer

2. Narrow band

Example : choke

Broad band balun Narrow band balun


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Presented By: Ms. Shail Bhatt

Practical Antenna

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