Antenna and Propagation
• Active and Passive Gain
• Azimuth and elevation charts (antenna radiation envelopes) • Interpreting polar charts
• Beamwidth • Antenna types
– Omnidirectional antennas – Semidirectional antennas – Highly directional antennas – Phased array antennas – Sector antennas
• Visual line of sight • RF line of sight • Fresnel zone • Earth bulge
• Antenna polarization • Antenna diversity
Signal and Antenna Concepts
• Signal must be radiate with enough power so that is will be understood by the
receiver
• Antenna installation will have a great effect.
– Simple-omni-directional in middle of office – Complex-multiple semi-directional antennas
Active and Passive Gain
• Measurements of the increase in signal from the Antenna (EIRP)
– If gain is from an amplifier or increased power at the transmitter-it is ACTIVE Gain
– If from shaping or focusing the power with an antenna,-PASSIVE Gain
Passive Gain
• Focusing isotropic energy in a specific pattern
• Created by the design of the antenna
Active Gain
• Providing an external power source
– Amplifier
Azimuth and Elevation Charts
• Charts help you “see” how antennas focus energy
– Polar charts or antenna radiation envelopes
Azimuth and Elevation Charts
• In either chart, the antenna is placed at the middle of the chart.
– Azimuth chart = H-plane = top-down view – Elevation chart = E-plane = side view
• Like casting a shadow with your hand
Antenna Types
• Antennas not only amplify transmitted
signal, they amplify received signal
Omnidirectional Antennas
• Common on many access Points
– Dipole
• Rubber duck
• Bagel shaped transmission • Limited vertical coverage • Wide horizontal coverage
• Low gain antennas are usually 2.14dB
High Gain Omni-directional Antenna Low Gain Omni-directional Antenna
Provides 360º horizontal
coverage pattern along a flat plane.
Gain of signal along the
horizontal plane means less signal along the vertical plane
Omni-directional antennas are also known as dipoles.
Antennas
• Antennas are most effective when the element is an even fraction or a multiple of the wavelength (λ)
– ¼, ½, 1, 2, etc.
• Used in multipoint environments
– Indoor Access Point
– Building to multiple building coverage
Semidirectional Antennas
• Designed to direct the signal in a specific direction
– Point to point-outdoors about a mile – Down hallways
• Three types
– Planar Type
• Patch • Panel
– Yagi
Semidirectional Antennas
• Multiple planar antennas can be used to cover multiple aisles
– Libraries, Warehouses, Retail, etc
• Yagi antennas are like old TV antennas
– However, each element is fitted for wireless wavelengths
Patch Semi-directional Antennas
Patch, Panel, Yagi and Sector are the primary semi-directional antenna types on the market today
Semi-directional antennas have 180º or less of
horizontal and vertical beam width
Primary coverage uses include:
Hallways/Corridors
Wireless ISPs
PTP & PTMP Bridging
Multiple semi-directional antennas can be combined into an array to provide omni-directional coverage.
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Highly Directional Antennas
• For Point to Point
– Generally between two buildings
• Focused, narrow beamwidth • Two main types
– Parabolic Dish – Grid Antenna
• Since they are used outdoors, wind loading is an issue
Highly Directional Antennas
• Highly focused energy
Phased Array Antennas
• Multiple antennas connected to a signal processor
• Different antennas can be fed different phases
– Able to create very directed beams
• Usually regulated differently • Not common in 802.11
– 802.11n has a PHY specification that supports it
Sector Antennas
• Specialized high gain, semidirectional antenna
• Multiple antennas with limited back lobe
– Each antenna can be on its on its own transceiver
• Usually set up to provide 360 degree coverage
• Used extensively in cellular phone systems
Line of Sight
• The “line” from source to destination
– Visual LOS is not important on Wireless networks
• RF line of sight is
Maintaining Clear Communications
• Visual line of sightFresnel Zone
• Football shaped area around the “LOS” • Actually multiple zones surrounding the
main line of sight
• If first Fresnel zone is obstructed, it will affect the transmissions
Fresnel Zone
• No Fresnel math on test
• You need to understand how the
calculations will affect placement when designing a point to point.
• Can calculate the size of the zone in middle
– Or at certain distances
• Important for keeping out obstructions
Fresnel Zone
• Don’t mistake smaller beamwidth for smaller Fresnel zone
• Fresnel zone is affected by the frequency fo the transmission, not the antenna
Earth Bulge
• For longer Point to Point links-More than 7 miles
• Important to calculate height requirements for towers
Antenna Polarization
• Alignment of the antenna can affect polarization • Doesn’t matter if it is horizontal or vertical, both
transmitter and receiver need to be the same way
• Most systems have antenna diversity
– Multiple antennas (more than one wavelength away)
Antenna Diversity
• Helps compensate for multipath • An AP has multiple antennas
– Compares the signal from each antenna and choose the best signal
– Done frame bu frame
• 802.11n uses switched diversity
– Signal with the best amplitude is used – Also known as received diversity
• Can also use measurement to choose the transmit antenna
Antenna Diversity
• Don’t put the antennas in different
locations or point in different directions
– That defeats the purpose
• Usually a single radio with multiple connections
Multiple In Multiple Out (MIMO)
• More sophisticated type of antenna diversity
• Takes advantage of multipath • Uses Space Time Coding
– Send multiple signals simultaneously
Voltage Standing Wave Ratio
(VSWR)
• Measure of the change in impedances to an AC signal
– When there is a difference or mismatch in impedance between devices in a RF system
• Forward Energy is reflected backward to the transmitter
• Usually at points where tyou are connecting
Voltage Standing Wave Ratio
(VSWR)
• Ratio of energy reflected is the voltage reflection coefficient
– Return Loss
• Ideally, there is no mismatch
– A matched cable, 0 ratio, infinite return loss
• VSWR is a measure between the
maximum voltage and minimum voltage
• VSWR=Vmax/Vmin
Voltage Standing Wave Ratio
(VSWR)
• Higher the return loss, the less broadcast power
Signal Loss
• The goal when connecting the antenna to transmitter is to reduce signal loss
• Must pay attention to devices used
– Cables and connectors
Antenna Mounting
• Placement • Mounting
• Appropriate Use
• Orientation and alignment • Safety
Placement
• Correct placement for type of antenna
– Omnidirectional toward center
– Semi-direction at edge, pointed toward center
• Pay attention to vertical and horizontal coverage • Also, power
levels-– Too high a power will provide an overlarge coverage area
• Security risk
• Outdoors-Watch the Fresnel Zone
Mounting
• Outdoors
– Masts or towers
• Indoors
– Wall or ceiling
Appropriate Use
• Indoor vs. Outdoor
• Although they look the same, they are designed for correct temperature and environment
Orientation and Alignment
• Pay attention to horizontal vs. vertical alignment
• Polarization can make the difference
Safety
• Be careful
• RF health and safety courses
– FCC and OSHA regulations
• If installing on a tower, pole, etc-get a professional.
Maintenance
• Preventative and diagnostic • Don’t just set and forget
– Especially not outdoors
• Outdoors you need to be aware of wind and water damage
– Wind
• Properly mount
– Water
Antenna Accessories
• All devices attached in the RF system need to be checked for
– Frequency response – Impedance
– VSWR
– Maximum input power – Insertion loss
Cables
• Choose the correct cable based on technology, frequency, etc
– Some cables can’t be used with some frequencies
– Match the impedance
– Calculate the signal loss
• Different for different frequencies
– Purchase pre-cut of hire a professional
Connectors
• FCC has mandated that manufacturers
use unique connectors to limit the ability to use noncertified antennas
– Pigtail adapters get around this requirement
• Be careful of exceeding FCC regulations
RF Connectors
• Join cables and devices together
Splitters
• Signal splitter, RF splitter, Power Splitter • Used when you need the signal to go to
more than one location
– Multiple antennas
• Will cause loss and degradation of signal
– Power monitoring
Amplifiers
• For active gain • Unidirectional
– transmit only
• Bi-directional
– Transmit and receive
• Fixed output
– Generate a signal equal to that of the amplifier
• Fixed Gain
Attenuators
• In cases where you need to limit the range or power
– If the minimum power setting isn’t small enough
Lightning Arrestors
• Protect from power surges due to nearby strikes
– Direct strike will fry it
Lightening Arrestors
Does not prevent equipment loss in cases of direct lightning strikes!
Lightning Arrestors
An in-line RF device that must be connected to Earth ground
Dissipates static electricity in the air
When objects near RF antennas are struck,
Grounding Rod and Wires
• Important to have a common ground
– Properly sunk copper rod and connectors – Tinned copper wire to connect tower legs
