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Chapter 6
WLAN Antennas andAccessories
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ObjectivesUpon completion of this part, you will be able to:
Identify the basic attributes, purpose, andfunction of the different types of antennasOmni-directional/dipole
Semi-directionalHighly-directional
Describe the proper locations and methods forinstalling antennasExplain the concepts of polarization, gain,beam width, and free-space path lossIdentify the purpose of the different wirelessLAN accessories
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RF AntennasDevice used to convert high frequency electricalsignals on a transmission line (a cable or
waveguide) into propagated waves (RF) in the air.The physical dimensions of an antenna, aredirectly related to the operating frequency.
The electrical fields emitted from antennas arecalled beams or lobes .Three generic categories of RF antennas:
Omni-directionalSemi-directionalHighly-directional
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Dipole/Omni AntennaIsotropic radiator: radiates in all directionsequally (forming a sphere). Example??Omni Antenna: Produce 360-degree horizontalbeam in a shape of doughnut
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Dipole/Omni Antenna Radiation Pattern
Low gain Omni antenna:
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Dipole/Omni Antenna Radiation PatternHigh gain omni antenna:
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Dipole/Omni AntennaA common omni antenna is dipole antennaContaining one or more -wavelength elements
joined together.As the frequency gets higher, the wavelengthbecomes smaller. ( -wavelength at 2.4GHz is
about 2 inches)Radiated energy is concentrated into a doughnut region.
The standard antenna used for dBd measurement:A dipole antenna with a gain of 2.14dBi
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Structure of a typical dipole antenna
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Example Dipole/Omni Antennas
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Dipole/Omni AntennaScenario 1:
You will install Wireless LAN in a tradeshowhouse with 40-foot high ceiling.
What type of antenna should you choose?Where should you place the antenna?
High gain or low gain antenna is moresuitable, why?
4 0
f e e t
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Dipole/Omni AntennaScenario 2: Point-to-Multipoint Link (hub-n-spoke)
Large coverage areas are needed around acentral point
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Semi-directional AntennasDirect the energy from the transmittersignificantly more in one particular directionrather than the uniform, circular patternBack lobe and side lobes should be taken intoconsideration
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Structure of Yagi antenna
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Example Semi-directional Antennas
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Semi-directional AntennasIdeally used for short and medium range bridgingdown along hallways or corridors: up to 3.3 kmPoint-to Point wireless bridging:
Improve securityEliminate the need for multiple access points
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Semi-directional Antennas radiation pattern
Front lobe, back lobe, side lobes
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Semi-directional AntennasAzimuth and elevation charts:
A standard way of representing an antenna scoverage patternAzimuth chart shows a top-down view of the
antenna s coverage.
Elevation chart shows a side-view of theantenna s coverage.The rings represent different signal strengths
The external ring is normally marked 0dB
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Semi-directional Antennas
Azimuth chart:E-plane
Elevation charts:H-plane
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Highly Directional AntennasEmit the most narrow signal beamHave the greatest gainTypically concave, dish-shaped devicesIdeal for long distance, point-to-point wirelesslinks: 3 to 42 km
Must be mounted accurately to aim at each other
Radiation pattern:
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Highly Directional AntennasHighly-directional dishantenna
Beam width 4 - 5 degrees
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Highly Directional AntennasHighly-directional grid antenna
Beam width 5-10 degrees
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RF Antenna Concepts
PolarizationGainBeamwidthFree Space Path Loss
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PolarizationPolarization
Electro-magnetic field:A radio wave is made of uptwo fields, one electric andone magnetic which are onplanes perpendicular to each
otherPhysical orientation of theantenna along horizontal
or vertical plane: indicatesorientation of E-Plane
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Polarization
Vertical PolarizationHorizontal Polarization
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PolarizationHorizontal polarization- the electric field is parallel to the groundVertical polarization- the electric field is perpendicular to theground
If two antennas are not aligned correctly,what happens???
Sometime turning your laptop may havebetter receiving quality, why???
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PolarizationNo signal will be received!
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GainGain
Antenna gain is specified in dBi, which meansdecibels referenced to an isotropic radiatorAn isotropic radiator radiates power equally in
all directions simultaneously. Gain=0dBi
The gain is passive -No increasing to the input power, but rathershape the radiation field
The higher the antenna gain, the farther thewave will travel
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Gain
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Gain
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BeamwidthBeamwidth
A measure of the angle between the two half-power(-3 dB) points on either side of the mainlobeNarrowing, or focusing antenna beams
increases the antenna s gain (measured in dBi)
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BeamwidthEach type of antenna has different beamwidthspecifications
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Free-space Path LossFree-space Path Loss
As transmitting, the wave front end becomes widening
the loss incurred by an RF signal due to "signaldispersion" which is a natural broadening of the wavefrontPower level decreases at a rate inversely proportional to
the distance traveled and proportional to the wavelengthof the signal
1010 log20log2022 += d PathLoss
( d: travelled distance in m: wav e length in m )
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Antenna Distance
The 6-dB rule6 dB loss induced for a distance doubledEach 6 dB increase in EIRP equates to adoubling of covered rangeA 6-dB of reduction in EIRP translates into a
cutting of the range in half.
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Antenna DistanceThe table below is used to calculate the link budget at
2.4GHz
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Antenna Distance
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Antenna Distance
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Link budget/system operating margin
Refers to the calculation of the amount of excesssignal strength a station is receiving relative to its
receive sensitivity. (Expressed in dBm.)Receive sensitivity is the absolute weakest signalstrength that the radio can reliably receive with abit-error-rate remaining below a certain threshold.Useful for long-distance outdoor point-to-pointlinks, such as a wireless bridge link between twobuildings.
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Fade margin
In reality, the strength of an 802.11 signal won tstay constant.
When planning for a WLAN installation, it scommon to include a few extra dB of signalstrength to the link budget. This extra signalstrength is known as : fade marginFade margins of between 10 and 20dB arecommon for both indoor and outdoor installations.
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Antenna Mounts
Antennas can be mounted on:ceiling mount - hung from drop ceilingswall mountpillar mount- mounts flush to a perpendicular surfaceground plane - sits flat on the groundmast mount - mounts to a polearticulating mount - movable mast mount
chimney mountTripod-mast - sits on top of a tripod
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Antenna Mounts
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Antenna Mounts
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Antenna Installation/configuration
Proper orientation : same polarizationPrecise alignment : for best throughputSecure mountingAvoid conductive areasPrevent moisture
Avoid obstructionsBest coverage
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RF Antenna Safety
Factory manualsKeep distance from transmission area
Avoid metal obstructionsProfessional installerAvoid power lines: distance twice the antenna
heightUse grounding rods to protect antenna fromlighting strike
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WLAN Accessories
RF AmplifiersRF Attenuators
Lightning ArrestorsGround rods/wiresRF Connectors
RF CablesRF Splitters
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RF Amplifiers
RF AmplifiersTo increase the amplitude of an RF signal when
compensating for the loss either due to thedistance or the length of cableThe GAIN is measured in +dB
"phantom voltage" :Normally powered using DC voltage fed ontothe RF cable with a DC injector near the AP
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RF Amplifiers
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RF Amplifiers
RF amplifiers come in two types: unidirectionaland bi-directional (close to antenna)
Before selecting an antenna, must know:Impedance (ohms)Gain (dB)
Frequency response (range in GHz)VSWRInput and output (mW or dBm) specifications
FCC violations results in fines of $27,500 -$1,200,000
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RF Amplifiers
Fixed, linear RF amplifiers are recommendedPlaced close to the antenna
Mounted to a solid surface facing down to keepwater out
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RF AttenuatorsA device that causes precisely measured loss
(in dB) in an RF signalPlaced close to the antenna
RF Attenuators
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RF Attenuators
All attenuators should come with a calibration report and
certificate
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Lightning Arrestors
Lightning ArrestorsUsed to shunt transient current into the ground
that is caused by lightningProtecting wireless LAN hardware such as APs,
bridges
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Lightning Arrestors
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Lightning Arrestors
Cost will be between $50 - $150 for any brand
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Ground rods/wires
A ground system has two purposesTo provide a safe path for lightning s current to
travel to ground (the earth).To ensure that all connected electrical systemshave a common ground.
Grounding: driving a copper rod into the earth,then connecting equipment to that rod via low-impedance wires.The National Electrical Council (NEC) requires aminimum depth of 8 feet .
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RF Splitters
RF SplittersA device that has a single input connector and
multiple output connectorsPassive device
l
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RF SplittersChoosing an RF Splitter
Insertion loss:
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RF Connectors
RF connectors:Normally 50 ohm impedance
Insertion loss
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RF Cables
RF cablesLowest loss Cable within the budget
Impedance 50 ohmsFrequency response. E.g. a rating of at least2.5GHz for 2.4 GHz WLAN
VSWRRG-58 cable should not be used as extensions tothe antenna
RF C bl
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RF Cables
Three major manufacturers:Andrew: Heliax cable
Times Microwaves: LMR (Industry standard)
RF igt il Ad t C bl
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RF pigtail Adapter Cable
RF pigtail Adapter CableUsed to adapt proprietary connectors to
industry standard connectors like N-type andSMA connectors
Freq enc Con erter
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Frequency Converter
Frequency Converterused for converting one frequency range to
another for the purpose of decongesting afrequency band
Frequency Converter
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Frequency Converter
Example: Building-to-Building wirelessconnectivity to allocate more than 3 non-
overlapping connections
Bandwidth Control Unit
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Bandwidth Control Unit
BCU:Wireless LANs are a shared medium with very
low throughput in comparison with wired LANFor outdoor WISPs: Bandwidth should becontrolled such that each user has a reliableand consistent connectivity and gets what theyare paying forFor Indoor: to give indoor LAN users as muchbandwidth as possible by not overloading
access points
Operation of BCU
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Operation of BCU
Work by filtering on MAC addresses in order todrop each user into a pre-assigned queue
Each queue has particular properties such asupstream and downstream bandwidthMultiple users might be put into the same queuefor precise bandwidth control and accounting peruser
BCU Management Software
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BCU Management Software
Test Kits
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Test Kits
Test KitsUsed for testing cables and connectors
Consist of an RF signal generator and athrough-line power meter
Chapter 6 Notes
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Chapter 6 Notes
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