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

5

H Plane

6

V Plane

7

Beam width

8

Beam width

9

Types of Antenna

10

Antenna types

11

Patch antenna

12

Yagi antenna

13

Radiation pattern semi directional antenna

14

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

15

Adjusting Antennas: RF Transmissions (continued)

Figure 10-14: Radio frequency link

16

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

17

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

18

Link budget

19

Example (Specs)

20

Adjusting Antennas: RF Transmissions (continued)

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

21

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

22

Adjusting Antennas: Antenna Types (continued)

Figure 10-15: Rod antenna pattern

23

Omni Directional Antennas

24

Omni directional antennas

25

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

26

Phased array antenna

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

Without physical realignment or movement

27

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

28

The bigger the better

29

Adjusting Antennas: Antenna Types (continued)

Figure 10-17: Azimuth and elevation pattern

30

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

32

Attenuators

33

Amplifiers

34

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

35

Splitters

36

Lightning

37

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

38

Adjusting Antennas: Lightning Arrestor (continued)

Figure 10-18: Lightning arrestor

39

RP TCN Lightning arrestor

40

Ethernet lightning arrestors

41

Grounding arrestors

42

Lightining

43

Isolating with fiber

44

Protection with fiber

45

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