Report - Comm3
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Transcript of Report - Comm3
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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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Vertical (Marconi) Antenna contd
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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Vertical (Marconi) Antenna contd
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.