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8/3/2019 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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Basic Antenna Principles
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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Omni directional (Dipole)Antennas
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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Omni directional (Dipole)Antennas
Dipole Side-View
Coverage Area Top View
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Omni directional (Dipole)Antennas
Coverage Area High-gainSide View
Coverage Area Top View
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Omni directional (Dipole)Antennas
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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Omni directional (Dipole)Antennas
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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Semi-directional Antennas
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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Semi-directional Antennas
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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Basic Antenna Principles
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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Basic Antenna Principles
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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Basic Antenna Principles
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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Basic Antenna Principles
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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Basic Antenna Principles
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
Recommended