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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