Report - Comm3

8/3/2019 Report - Comm3

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Quarter Wave Antenna

Marconi antenna

Monopole antenna


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Monopole Antenna

Vertical

Half the length of a dipole (one-quarter waveapproximately)

Ground supplies the other half If installed above ground, a ground plane can

be used instead

For a car antenna, the car is the ground plane

Input impedance half that of a dipole, about35 ohms


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1/4 wavemonopolewithgroundplanefor 144 MHz


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Monopole Antenna

wavelength fed at one end

Fed with unbalanced feed line with groundconductor connected to earth ground.

In practice it usually requires an array of radials todevelop a better ground plane. (Marconiantenna)

When used at low frequencies the field should be

vertically polarized and antenna could be a tower. The tower is ground insulated and fed at a point

above ground with a Gamma match. Z increasesupward.


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6

VerticalAntennasA vertical antenna is an antenna that consists of asingle element mounted perpendicular to the Earth'ssurface. Most mobile antennas are verticals.

Verticals usually require some sort of counterpoise towork their best. In a fixed station, a vertical may eitherbe mounted on the ground or on a mast, and it mayalso have several radials for counterpoise. Theseradials may be laid out on the ground, as in the nextslide, or mounted just underneath the vertical element,as in an elevated ground plane.

In a mobile installation, the metal body of the car

usually serves as the counterpoise.


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Vertical (Marconi) Antenna

Vertical Antennas are used for frequencies

under 2 MHz. It uses a conducting path to

ground that acts as wavelength portion the

antenna above the ground. The above ground

structure represents a P/4 wavelength

An advantage for vertical polarization because

it can provide omnidirectional

communications


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Vertical (Marconi) Antenna contd

It radiates energy equally in all directions

(omnidirectional)

A quarter-wave antenna operates with oneend grounded.

Quarter-wave antennas are used both below

and above 2 MHz


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The whip antenna provides an omnidirectional

radiation pattern. It is a quarter-wave antenna

used for man pack and vehicular operations

The ground-plane antenna is a vertical

quarter-wave antenna that increases the

range of tactical FM radio sets. Its radial

elements provide a counterpoise that

simulates a ground.


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Marconi Antenna

Monopole / Marconi Antenna a one-

quarter wavelength long, mounted vertically

with the lower end either connected to

ground or grounded through the antenna

coupling network

Advantage: long

Disadvantage: Close to ground


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Poorgroundingconditionsofthe

earth/soilsurroundingtheantennacan

resultinserioussignalattenuation.Thisproblemisalleviatedbyinstallinga

counterpoise


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Counterpoise

Counterpoise isagroundinggrid

establishedwheretheearthgrounding

cannotsatisfyelectricalrequirementsforcircuitcompletion.Itisdesignedtobe

non-resonantattheoperatingfrequency


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Counterpoise-contd

supports

antenna

radius = P


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Radiation Pattern

Radiationpatternisanindicationof

radiatedfieldstrengtharoundthe

antenna.


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Radiation Pattern for Vertical

Antennas

antenna

P/4

P/2


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Vertical Antenna Design

Many vertical antennas are designed to be equalin length to one-quarter wavelength of thedesired operating frequency.

For a 1/4 wave vertical:

234

Length (feet) = ----------------------

Frequency (MHz)


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Sample Problem

For example, suppose you want to know theapproximate length, in inches, of a quarter-wavelengthvertical antenna for 146 MHz.

Using the formula in the previous slide:

234

Length = ------ = 1.6 feet

146

To get inches, multiply 1.6 times 12 (since there are 12inches in a foot) to get 19.2 or about 19 inches.


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Antenna Element


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Antenna Element

Antenna element - isanindividualradiator,

suchasahalf- orquarter-wavedipole.

Individualantennasarecalledelements

Types of Antenna Elements:

1. Driven2. Parasitic(Nondriven)


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Types of Antenna Elements: Driven Elements aredirectlyconnectedto

thetransmissionlineandreceivepowerfrom

oraredrivenbythesource.

Parasitic (Nondriven) arenotconnectedtothetransmissionline;theyreceiveenergy

onlythroughmutualinductionwithadriven

elementoranotherparasiticelement.

- someelementsnotconnectedtosource

- theyre-radiatepowerfromotherelements


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Parasitic contd

Reflector longerthanthedrivenelementfrom

whichit receivesenergy.

- iteffectivelyreducesthesignalstrengthin

its directionandincreasesitintheopposite

direction.

Director - shorterthanitsassociateddriven

element.- increasesthefieldstrengthinitsdirection

and reducesitintheoppositedirection.

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