CSNT 180 Wireless Networking. Chapter 6 WLAN Antennas and Accessories

Transcription

1 CSNT 180 Wireless Networking Chapter 6 WLAN Antennas and Accessories Norman McEntire norman.mcentire@servin.com Founder, Servin Corporation, Technology Training for Technology Professionals TM Copyright (c) 2010 Servin Corporation. 1

3 Introduction Antennas are important to WLANs Remember the iphone Antennagate! Transmitter view of antenna Transform AC signals from hardware and transform into radio waves Receiver view of antenna Receive radio waves and transform into AC signals for the hardware Copyright (c) 2010 Servin Corporation 3

5 Basic RF Antenna Concepts Important to understand antenna terminology RF Lobes shape of the RF patterns Beamwidth horizontal and vertical measurement angles Antenna charts Azimuth [az-uh-muhth] or Horizontal Elevation Gain changing the RF coverage Polarization horizontal or vertical Copyright (c) 2010 Servin Corporation 5

6 RF Lobes RF Lobe = shape of the RF energy emitted from the antenna RF Lobes are determined by the physical design of the antenna See Figure 6.1 Type of antenna determines the RF lobe shape Omnidirectional Semidirectional Highly Directional Parabolic Disk Copyright (c) 2010 Servin Corporation 6

7 Beamwidth Beamwidth = angle of measurement of the main RF lobe measured at half-power point or -3 db point Measured in degrees both horizontally and vertically Figure 6.2 Azimuth [az-uh-muhth] and Elevation charts for antennas will show beamwidth angles Azimuth horizontal RF coverage (bird's eye view) Elevation vertical RF coverage (side view) Copyright (c) 2010 Servin Corporation 7

9 Antenna Gain Gain = change in coverage that is a result of the antenna focusing the area of RF propagation Amplification of an RF signal will result in gain Antenna gain measured in dbi dbi = decibels isotropic Change in power as result of increasing the isotropic energy Isotropic energy = energy emitted equally in all directions See Figure 6.3 Copyright (c) 2010 Servin Corporation 9

10 Passive Gain Passive Gain = a change in coverage without the use of an external power source Antennas focus isotropic energy into a specific radiation pattern AC signal comes from the hardware Antenna provides passive gain, increasing the power output See Figure 6.4 Copyright (c) 2010 Servin Corporation 10

12 Antenna Polarization Antenna Polarization How the RF wave emitted from the antenna The orientation of the electrical component or electric field of the waveform To maximize signal Transmitting antenna and receiving antenna should be polarized in the same direction Signal decreases when antennas have different polarization Figure 6.5 Copyright (c) 2010 Servin Corporation 12

13 InSSIDer Use InSSIDer (included on CD that comes with book) for Antenna Polarization Example/Experiment InSSIDer displays the received signal strength from the APs in the receiver area RSSI Received Signal Strength Page 166 in book Copyright (c) 2010 Servin Corporation 13

15 WLAN Antenna Types The type of antenna that is best for a given site depends on the desired RF coverage Good WLAN design requires good antenna choice If wrong antenna selected Interference Poor signal strength Less than ideal coverage Copyright (c) 2010 Servin Corporation 15

17 Omnidirectional Antennas The most common antenna for indoor SOHO and Enterprise APs Horizontal beamwidth (azimuth) of 360 degrees Vertically polarized perpendicular to earth surface Horizontal radiation pattern is 360 degrees Propagate RF energy in every direction See Figure 6.6 Observe donut shape Copyright (c) 2010 Servin Corporation 17

18 2.4 GHz Rubber Duck Antenna See Figure 6.7 Very common omnidirectional antenna Pivot point so the polarization can be adjusted horizontal or vertical, regardless of how mounted Low gain: 2 dbi to 3 dbi Copyright (c) 2010 Servin Corporation 18

19 Omnidirectional Antenna Specifications Electrical Specifications Frequency Range: MHz Gain: 2.2 dbi Horizontal Beamwidth: 360 degrees Impedance: 50 ohms Impedance = measurement of AC resistance Maximum Power: 50 W VSWR (Voltage Standing Wave Ratio) < 2:0 VSWR = disruption common in WLAN caused by impedance mismatch Copyright (c) 2010 Servin Corporation 19

21 Semidirectional Antennas Semidirectional antennas focus power into a more specific pattern than omnidirectional Horizontal radiation pattern less than 360 degrees Various types Patch, Panel, Sector, Yagi [yah gee] Often used outdoors or indoor rectangular areas Often used for outdoor point-to-point and pointto-multipoint Copyright (c) 2010 Servin Corporation 21

23 Appropriate Use of Semidirectional Antenna See Page 173 Real World Scenario Top Figure The rectangular office area Bottom Figure Solution with Omnidirectional antenna on APs spread throughout Figure on Next Page Solution with Patch antenna with AP on both ends Copyright (c) 2010 Servin Corporation 23

26 Sector Antennas Used to create omnidirectional radiation pattern using semidirectional antennas Horizontal (azimuth) various from degrees Commonly configured in an array Ex: Three each with azimuth of 120 = 360 Figure GHz 14 dbi 90 degree sector panel antenna Copyright (c) 2010 Servin Corporation 26

28 Yagi [yah gee] Antennas Indoors For use in long hallways and corridors Outdoors Short-range bridging (less than 2 miles) Vertical and horizontal beamwidths from Figure 6.14 NOTE: Given a figure, be sure to identify the antenna Copyright (c) 2010 Servin Corporation 28

30 Highly Directional Antennas Typically parabolic disk antennas for long-range point-to-point bridging Range of 25 miles or more depending on gain and environmental conditions Narrow Beamwidths from 3-15 degrees Figure 6.16 Copyright (c) 2010 Servin Corporation 30

31 Highly Directional Antenna Specs Table 6.5 Specs Electrical Mechanical Shipping Specs Wind Loading Figure 6.17 Charts Vertical (elevation) Horizontal (azimuth) Copyright (c) 2010 Servin Corporation 31

33 RF Cables and Connectors RF cables and connectors may be used to connect antennas GOAL: Minimize decrease in signal strength Factors include Type Length Cost Impedance Impedance = measure of AC resistance Copyright (c) 2010 Servin Corporation 33

34 Cable Types Cables often used to extend distance from AP to external antenna WLANs use 50 ohm cable (CATV uses 75 ohm cable) Use rigid cable for long runs Figure 6.18 Copyright (c) 2010 Servin Corporation 34

35 Cable Length All cable (even very short) will have some signal loss Best practices Use correct length cable Not too long. Not too short. Just right. Minimize connections Pigtail short cable to connect standard cable to proprietary cable Figure 6.19 Copyright (c) 2010 Servin Corporation 35

37 Impedance and VSWR Impedance = measure of AC resistance Rule: Match impedance of all components to minimize signal loss VSWR Voltage Standing Wave Ratio Impedance mismatch results in VSWR High level of VSWR impacts WLAN performance WLANs have impedance of 50 ohms Copyright (c) 2010 Servin Corporation 37

38 RF Connectors RF connectors join devices together AP to Antenna Antenna to Cable Cable to Cable Connectors result in insertion loss Remember: To minimize VSWR keep connections to a minimum Figure 6.20 Common RF connectors Copyright (c) 2010 Servin Corporation 38

39 Proprietary Connectors Some regulatory domains require the use of proprietary connectors on antenna Use to prevent using the wrong antenna and exceeding the maximum power level See MC, MMCX, MCX, and RP-MMCX connectors by vendors Bottom of Page 186 Copyright (c) 2010 Servin Corporation 39

41 Factors in Antenna Installation Factors in Antenna Installation Earth Curvature Antenna Placement Multipath Omnidireciontal Placement Semidirectional Placement Highly Directional Placement RF Line of Sight Copyright (c) 2010 Servin Corporation 41

42 Addressing the Effects of Earth Curvature Earth Curvature sometimes called Earth Bulge After 7 miles, the curvature of the earth has an impact on point-to-point WLAN links Must add height to antenna to compensate for earth curvature A formula used to calculate, but not covered by book, nor needed for CWTS exam Copyright (c) 2010 Servin Corporation 42

44 Omnidirectional Placement Common to attach omnidirectional antenna direct to AP High-gain omnidirectional may be used for outdoor service Outdoor placement depends on coverage area and gain of antenna Copyright (c) 2010 Servin Corporation 44

45 Semidirectional Placement May be used for indoor or outdoor installations Patch/Panel antenna typically mounted flat on a wall Yagi antenna mounted for point-to-point Copyright (c) 2010 Servin Corporation 45

47 Reflection and Multipath Reflection RF signal bouncing off smooth, non absorptive surface Reflection caused by RF signal bouncing off walls, ceilings, floors, furniture, etc. The effect of reflection is a decrease in signal strength due to a phenomenon called multipath Multipath is the result of several wavefronts of same transmission received out of phase at different times Fig 6.21 Copyright (c) 2010 Servin Corporation 47

48 Antenna Diversity Use Antenna Diversity to reduce effects of multipath Use 2 antennas with 1 radio to decrease multipath Receive signal on both antenna and choose the best signal Typically spaced 1 wavelength apart Antennas must be Same design, Frequency, Gain, etc. Oriented the same polarization Copyright (c) 2010 Servin Corporation 48

50 Combating Effects of Wind and Lightning on Wireless Connections Most weather conditions do not affect WLAN Rain, Snow, Sleet Two that do impact WLAN Wind Use Wind Loading Data Lightning Use Lighting Arrestor Copyright (c) 2010 Servin Corporation 50

51 Lighting Arrestors Lighting Arrestors In-series device installed after the antenna and prior to transmitter/receiver Protects from indirect lighting strike by shunting excess current to ground No protection from direct lighting strike Figure 6.22 Copyright (c) 2010 Servin Corporation 51

52 Grounding Rods Grounding rod A metal shaft used for grounding a device Drive rod into ground at least 8 feet NOTE: WLAN Engineer does not typically install the Grounding Rod, but rather a professional contractor Copyright (c) 2010 Servin Corporation 52

54 Installation Safety Installing WLAN Antennas may require: Professional Contractors Bonding Certifications Local Building Code Approvals/Inspections Remember Avoid power lines Use safety equipment when working at heights Use grounding rods when appropriate Copyright (c) 2010 Servin Corporation 54

56 Antenna Mounting Step 1 is to choose the proper antenna Step 2 is to correctly mount the antenna Depends on antenna type Depends on indoor/outdoor mounting Depends on use (point-to-point vs AP/client) Common Mounting Types Pole/Mast Mount Ceiling Mount Wall Mount Copyright (c) 2010 Servin Corporation 56

59 Wall Mount When mounting to wall be sure to consider the polarization of the antenna Remember Some antennas designed for wall mounting Some antennas designed for ceiling mounting Copyright (c) 2010 Servin Corporation 59

63 Fresnel Zone Fresnel [fruh-nel] Zone Area of RF coverage surrounding the Visual LoS Best Practice Maintain an obstruction-free clearance of at least 60 percent for the Fresnel zone to have acceptable RF LoS Figure 6.25 Copyright (c) 2010 Servin Corporation 63

64 Antenna Summary Lobes Beamwidth Passive Gain, Active Gain Vertical and Horizontal Polarization Omnidirectional, Semidirectional, Highly Directional Visual LoS, RF LoS Copyright (c) 2010 Servin Corporation 64