Chapter 02 - Antennas

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ITNW 1351Fundamentals of Wireless

LANsChapter 2

Antennas


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Antennas

Antennas are most often used toincrease the range of a wireless LANsystem

Proper antenna selection can alsoenhance security of a wireless LAN

are most sensitive to RF signalswhose wavelength is an even multipleof the antennas length (includingfractional multiples such as or )


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Basic Antenna Principles

1. Antennae convert electrical energyto RF waves in the case oftransmitting antennae

OR

RF waves into electrical energy inthe case of receiving antennae

2. The physical dimensions (especiallylength) of an antenna are directlyrelated to the frequency at whichthe antenna can propagate or

receive waves


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3. The physical structure of anantenna is directly related to theshape of the area in which it

concentrates most of its radiated RFenergy


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Generic Categories of RFantennas

Omni directional

Semi-directional

Highly-directional

Each category has multiple types of

antennas, each having different RFcharacteristics and appropriate uses


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Omni directional (Dipole)Antennas

Most common wireless LAN antennais a dipole

Standard equipment on most accesspoints

Radiates energy equally in all directionsaround its axis


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Radiates in a 360-dgree horizontalbeam Sphere = isotropic radiator sun =

example only theoretical Radiates in all directions around axis,

but does not radiate along the length ofthe wire itselfLooks like a donutThe higher the gain, the more the donut is

squeezed until it looks like a pancake


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Dipole Side-View

Coverage Area Top View


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Coverage Area High-gainSide View

Coverage Area Top View


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If placed in center of a single floor ofa multistory building, most energywill be radiated along the length of

that floor, with some sent to thefloors above and below High-gain omnis offer more horizontal

coverage area, but vertical coverage is

reduced Important consideration when mounting

on high indoor ceiling


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used when coverage in alldirections around the horizontal isrequired

most effective where large coverageareas are needed around a central point

commonly used for point-to-multipointdesigns with star topology

outdoors should be placed on top ofstructure in the middle of the coveragearea


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Passive Gain Antennas use passive gain

Total amount of energy emitted byantenna doesnt increase only thedistribution of energy around the

antenna Antenna is designed to focus more

energy in a specific direction

Passive gain is always a function of theantenna (i.e. independent of thecomponents leading up to the antenna


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Passive Gain Advantage

Does not require external power

Disadvantage

As the gain increases, its coveragebecomes more focused

Highest-gain antennas cant be usedfor mobile users because of their

tight beam

Active gain involves an amplifier


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Omni directional Antenna Usage

Used when coverage is required in alldirections around the horizontal axis

Most effective when large coverageareas are needed around a centralpoint

Commonly used for point-to-multipointdesigns


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Omni directional Antenna Usage

2 to 5 dBi treat as an isotropic radiator

Signal above and below center line will

be weaker 5 to 8 dBi

Appropriate for mounting above users

Increase in gain means decreasedvertical tolerance

8 to 10 dBi

Very flat


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Semi-directional Antennas

Direct energy from the transmittersignificantly more in one particulardirection

Often radiate in a hemispherical orcylindrical coverage pattern

Have back and side lobes that, ifused effectively, may further reducethe need for additional access points


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Frequently used types: Patch & Panel flat, designed for wall

mounting

Focus coverage in horizontal arc of 180 orless

Yagi elongated, ribbed, and usuallyhoused in an enclosure for moisture

protectionCommon vertical & horizontal beamwidths

of 90 or less

30 or less average


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Directional PatchAntenna

DirectionalYagi Antenna


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Beamwidth

Calculated by measuring the numberof degrees off-axis where beamdrops to (3 dB) of strength at the

0 position

-3 dB

-3 dB

Beamwidth(degrees)


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Beamwidth

Two vectors must be considered when discussingan antennas beamwidths

Horizontal = parallel to Earth

Vertical = perpendicular to Earth

HorizontalBeamwidth

Vertical

Beamwidth


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Beamwidth

AntennaType

HorizontalBeamwidth

VerticalBeamwidth

Omni 360 7 to 80

Patch/Panel 30 to 180 6 to 90

Yagi 30 to 78 14 to 64

ParabolicDish

4 to 25 4 to 21


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Azimuth & Elevation Charts

Provide a more accurate picture ofantennas beamwidth

Standard way of representingcoverage pattern

Azimuth = top-down view

Elevation = side-view

See Text


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Semi-directional Antenna Usage

Ideally suited for short and medium-rangebridging In some cases, semidirectional antennas

provide such long-range coverage that they

eliminate the need for multiple access points ina building

Patch or panel typically used on short rangebuilding-to-building & in-building directional

links Yagis most often used on short to mediumlength building-to-building bridging up to 2miles (3.3 km)


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Highly-Directional Antennas

High-gain antennas that emit the mostnarrow signal beam of any antenna type

Greatest gain of any of the types

Typically concave, disk-shaped devices(similar to satellite TV antenna)

Parabolic dishes

Some are grids (grid antennas) providesgood resistance to wind loading

Ideal for long distance, point-to-pointwireless links


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Highly-Directional AntennaRadiation Pattern

Highly-directional antennas are neverappropriate for mobile users


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Highly-Directional Antennas

These are not for clients usage Used for point-to-point communication

links

Have a very narrow beamwidth and must beaccurately aimed at each other

May be aimed directly at each other within abuilding in order to blast through an RF

signal absorbing obstruction

Can transmit at distances up to 58 km(about 35 miles)


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Line of Sight (LOS) = the apparentlystraight line from the transmitter tothe receiver

Why apparently straight?Remember refraction, diffraction, &

reflection?

Can be affected by blockage of theFresnel Zone


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Fresnel* Zone = an area centered on thevisible LOS between the transmitting andreceiving antenna

It defines an area around the LOS thatcan introduce RF signal interference ifblocked

As an obstacle obstructs the zone,energy is absorbed and prevented fromgetting to the receiver

*fr-nl'


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Another way of defining the FresnelZone is a series of concentricellipsoid-shaped areas around the

LOS path

LOSFresnel

ZoneTX RX


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Fresnel Zone 20 to 40 % blockage introduces little to

no interference into the link

Best to allow no more that 20 %blockage

>40 % means link will be unreliable Usually not encroached indoors unless

signal is partially or fully blocked

Constantly changes in mobileenvironment

Users dismiss it to simply bad

coverage


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Fresnel ZoneFormula to calculate the 60 % unobstructed

(minimum clear) radius around the visual LOS

r = 43.3* Xd/4f

r = radius in feet

d = link distance in milesf = frequency in GHz

The radius is 60 % smaller than the radius of theentire zone.

*Substitute 72.2 to calculate the radius of theFresnel Zone itself

The beamwidth is NOT a factor in calculating


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Antenna Gain = results from focusingthe RF radiation into a tighter beamwhich creates a seemingly brighter

beam Example: radiating at 30 degrees rather

that 360 degrees at the same power

the beam will radiate farther


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Intentional Radiator (IR) = an RF devicespecifically designed to generate andradiate RF signals

Includes:

RF device

All cabling

All connectors up to but not includingthe antenna

Any reference to power output of the IRrefers to power at the last connectorbefore the antenna

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Chapter 02 - Antennas