Antenna Fundamentals Lecture 4. 2 Objectives List some of the characteristics of RF antenna transmissions Describe the different types of antennas.

Antenna Fundamentals

Lecture 4

2

Objectives

• List some of the characteristics of RF antenna transmissions

• Describe the different types of antennas

3

Antenna principles

4

Adjusting Antennas: Antenna Types (continued)

Figure 10-17: Azimuth and elevation pattern

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H Plane

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V Plane

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Beam width

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Beam width

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Types of Antenna

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Antenna types

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Patch antenna

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Yagi antenna

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Radiation pattern semi directional antenna

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Adjusting Antennas: RF Transmissions

• May need to adjust antennas in response to firmware upgrades or changes in environment– May require reorientation or repositioning– May require new type of antenna

• Radio frequency link between sender and receiver consists of three basic elements:– Effective transmitting power– Propagation loss– Effective receiving sensibility

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Adjusting Antennas: RF Transmissions (continued)

Figure 10-14: Radio frequency link

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Adjusting Antennas: RF Transmissions (continued)

• Link budget: Calculation to determine if signal will have proper strength when it reaches link’s end– Required information:

• Antenna gain

• Free space path loss

• Frequency of the link

• Loss of each connector at the specified frequency

• Number of connectors used

• Path length

• Power of the transmitter

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Adjusting Antennas: RF Transmissions (continued)

• Link budget (continued):– Required information (continued):

• Total length of transmission cable and loss per unit length at specified frequency

• For proper WLAN performance, link budget must be greater than zero– System operating margin (SOM)– Good WLAN link has link budget over 6 dB– Fade margin: Difference between strongest RF

signal in an area and weakest signal that a receiver can process

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Link budget

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Example (Specs)

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Adjusting Antennas: RF Transmissions (continued)

• Attenuation (loss): Negative difference in amplitude between RF signals– Absorption– Reflection– Scattering– Refraction– Diffraction– Voltage Standing Wave Ratio

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Adjusting Antennas: Antenna Types

• Rod antenna: Antenna typically used on a WLAN– Omnidirectional– 360 degree radiation pattern– Transmission pattern focused along horizontal plane– Increasing length creates “tighter” 360-degree beam

• Sectorized antenna: “Cuts” standard 360-degree pattern into four quarters– Each quarter has own transmitter and antenna– Can adjust power to each sector independently

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Adjusting Antennas: Antenna Types (continued)

Figure 10-15: Rod antenna pattern

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Omni Directional Antennas

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Omni directional antennas

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Adjusting Antennas: Antenna Types (continued)

• Panel antenna: Typically used in outdoor areas– “Tight” beamwidth

• Phase shifter: Allows wireless device to use a beam steering antenna to improve receiver performance – Direct transmit antenna pattern to target

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Phased array antenna

: Incorporates network of phase shifters, allowing antenna to be pointed electronically in microseconds,

Without physical realignment or movement

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Adjusting Antennas: Antenna Types (continued)

• Radiation pattern emitting from antennas travels in three-dimensional “donut” form– Azimuth and elevation planes

• Antenna Accessories:– Transmission problem can be resolved by adding

“accessories” to antenna system– Provide additional power to the antenna, decrease

power when necessary, or provide additional functionality

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The bigger the better

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Adjusting Antennas: Antenna Types (continued)

Figure 10-17: Azimuth and elevation pattern

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Adjusting Antennas: RF Amplifier

• Increases amplitude of an RF signal– Signal gain

• Unidirectional amplifier: Increases RF signal level before injected into transmitting antenna

• Bidirectional amplifier: Boosts RF signal before injected into device containing the antenna– Most amplifiers for APs are bidirectional

31

Adjusting Antennas: RF Attenuators

• Decrease RF signal– May be used when gain of an antenna did not match

power output of an AP

• Fixed-loss attenuators: Limit RF power by set amount

• Variable-loss attenuators: Allow user to set amount of loss

• Fixed-loss attenuators are the only type permitted by the FCC for WLAN systems

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Attenuators

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Amplifiers

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Adjusting Antennas: Cables and Connectors

• Basic rules for selecting cables and connectors:– Ensure connector matches electrical capacity of

cable and device, along with type and gender of connector

– Use high-quality connectors and cables – Make cable lengths as short as possible– Make sure cables match electrical capacity of

connectors– Try to purchase pre-manufactured cables – Use splitters sparingly

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Splitters

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Lightning

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Adjusting Antennas: Lightning Arrestor

• Antennas can inadvertently pick up high electrical discharges – From nearby lightning strike or contact with high-

voltage electrical source• Lightning Arrestor: Limits amplitude and

disturbing interference voltages by channeling them to ground– Designed to be installed between antenna cable and

wireless device• One end (3) connects to antenna• Other end (2) connects to wireless device• Ground lug (1) connects to grounded cable

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Adjusting Antennas: Lightning Arrestor (continued)

Figure 10-18: Lightning arrestor

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RP TCN Lightning arrestor

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Ethernet lightning arrestors

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Grounding arrestors

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Lightining

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Isolating with fiber

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Protection with fiber

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Summary (continued)

• An antenna is a copper wire or similar device that has one end in the air and the other end connected to the ground or a grounded device

• There are a variety of characteristics of RF antenna transmissions that play a role in properly designing and setting up a WLAN

46

AP 1200 initial configuration sample

config t

hostname "raul"

interface dot11radio 0

ip address 10.0.0.1 255.255.255.0

no ip route-cache

channel "Channel 1-11"

ssid "raul"

authentication open

guest-mode

End

write

47

Lab 3

• 2.5, 3. 4 and 3.5 of Text Book