CCNA Wireless Official Exam Certification Guide part 12 docx

82 CCNA Wireless Official Exam Certification Guide 13.5 Yagi Antenna The 13.5 yagi antenna is a directional antenna that offers a very direct radiation pattern. Sometimes you see these mounted above doorways to cover a long hallway. You can also put a number of them side by side on a wall to cover a large open space such as a ware- house or convention center. Yagi antennas are sometimes called Yagi-Uda antennas, after their two creators. Note: For more information on the history of the Yagi-Uda antenna, see http://en. wikipedia.org/wiki/Yagi_antenna#History. Yagi antennas have a butterfly effect that is an effect of their polarization, as illustrated in Figure 5-17. You can clearly see the butterfly-type pattern in the figure. Notice that there is also some coverage on the back side of the antenna, even though it is designed as a directional an- tenna. This fact can be useful if you want to test under the antenna. Figure 5-18 shows the 10-dBi yagi, one of the yagi antennas offered by Cisco. Although it is enclosed in an aesthetically pleasing cylinder, the antenna inside is a “comb” that resembles old UHF television antennas that you used to see on the roofs of houses. –35 –30 –25 –20 –15 –10 –5 0 30330 60300 90270 120 240 150210 180 Figure 5-16 E-plane of the AIR-ANT-2485P-R 06_1587202115_ch05.qxp 9/29/08 2:45 PM Page 82 Chapter 5: Antenna Communications 83 Directional Yagi 28–80 Degrees at 2.4 GHz 68–78 Degrees at 900 MHz Figure 5-17 Radiation Pattern of a Yagi Antenna Figure 5-18 AIR-ANT2410Y-R Figure 5-19 AIR-ANT1949 Yagi Figure 5-19 shows the AIR-ANT1949, another yagi antenna offered by Cisco. This yagi is a high-gain antenna at 13.5 dBi. Its H-plane and E-plane are shown in Figures 5-20 and 5-21, respectively. Table 5-6 shows the details of the AIR-ANT1949 yagi. Key Topi c 06_1587202115_ch05.qxp 9/29/08 2:45 PM Page 83 84 CCNA Wireless Official Exam Certification Guide Table 5-6 AIR-ANT1949 Yagi Frequency range 2.4 to 2.83 GHz Gain 13.5 dBi Polarization Ver t i c a l H-plane 30 degrees E-plane 25 degrees Antenna connector type RP-TNC Mounting Mast/wall mount *This connector type is covered later, in the section “Antenna Connectors and Hardware.” Key Topi c –35 –30 –25 –20 –15 –10 –5 0 30330 60300 90270 120 240 150210 180 Figure 5-20 H-plane of the AIR-ANT1949 Yagi 06_1587202115_ch05.qxp 9/29/08 2:45 PM Page 84 Chapter 5: Antenna Communications 85 –35 –30 –25 –20 –15 –10 –5 0 30330 60300 90270 120 240 150210 180 Figure 5-21 E-plane of the AIR-ANT1949 Yagi When you mount a yagi, the polarity is important to consider. Because the antenna is en- closed in a protective casing, you might not be able to look at it and know the radiation pattern. Read the manufacturer documentation or look for manufacturer markings that in- dicate how to mount the antenna. On a Cisco yagi, the bottom usually is indicated by a black dot. Remember that if you mount it incorrectly, you will degrade the signal. 21-dBi Parabolic Dish The 21-dBi parabolic dish antenna, shown in Figure 5-22, is almost 100 times more pow- erful than the rubber duck (discussed in the section “2.2-dBi Dipole”). Parabolic dish antennas have a very narrow path. Their radiation pattern is very focused. When you install these, you have to be very accurate in the direction you point them. You would use a parabolic dish in point-to-point scenarios. Distances of up to 25 miles at 2.4 GHz and 12 miles at 5 GHz can be reached using parabolic dish antennas. Parabolic dish antennas have a butterfly effect similar to yagi antennas. Also, some parabolic dish anten- nas allow polarity to be changed. This is important, because they can be mounted at dif- ferent angles, and polarity changes how the RF is propagated. Table 5-7 shows the details of the Cisco AIR-ANT3338. 06_1587202115_ch05.qxp 9/29/08 2:45 PM Page 85 86 CCNA Wireless Official Exam Certification Guide Dual-Patch “Omnidirectional” 5.2 dBi, Pillar Mount Another special type of antenna to consider is the dual-patch 5.2-dBi pillar-mount omni- directional, shown in Figure 5-23. It is considered “special” because it has two patch directional antennas placed back to back, making it “omnidirectional.” Because there are actually two antennas, you can use diversity with this antenna. Table 5-7 AIR-ANT3338 Parabolic Dish Antenna PPoowweerr 5 Watts GGaaiinn 21 dBi PPoollaarriizzaattiioonn Ver t i c a l HH ppllaannee 12 degrees EE ppllaannee 12 degrees AAnntteennnnaa ccoonnnneeccttoorr ttyyppee RP-TNC MMoouunnttiinngg Mast mount *This connector type is covered later, in the section “Antenna Connectors and Hardware.” Key Topi c Figure 5-22 Parabolic Dish Antenna 06_1587202115_ch05.qxp 9/29/08 2:45 PM Page 86 Chapter 5: Antenna Communications 87 You would use this type of antenna to provide access to a hall, because it’s usually mounted to a pillar in the middle of the hall. Figures 5-24 and 5-25 show this antenna’s radiation patterns. Figure 5-23 AIR-ANT3213 Dual-Patch 5.2-dBi Pillar-Mount Omnidirectional Horizontal (H) Plane Pattern Elevation (E) Plane Pattern –35 –30 –25 –20 –15 –10 –5 0 30330 60300 90270 120 240 150210 180 Figure 5-24 AIR-ANT3213 Left Antenna Radiation Pattern 06_1587202115_ch05.qxp 9/29/08 2:45 PM Page 87 88 CCNA Wireless Official Exam Certification Guide Table 5-8 AIR-ANT3213 Antenna Frequency range 2.4 to 2.83 GHz Gain 5.2 dBi Polarization Ver ti c a l H-plane Omnidirectional E-plane 25 degrees Antenna connector type RP-TNC *This connector type is covered in the next section. Key Topi c In these two figures, the outer line is the H-plane, and the inner, dashed line is the E-plane. Table 5-8 shows the details of the AIR-ANT3213. Horizontal (H) Plane Pattern Elevation (E) Plane Pattern –35 –30 –25 –20 –15 –10 –5 0 30330 60300 90270 120 240 150210 180 Figure 5-25 AIR-ANT3213 Right Antenna Radiation Pattern 06_1587202115_ch05.qxp 9/29/08 2:45 PM Page 88 Chapter 5: Antenna Communications 89 Antenna Connectors and Hardware Cisco uses a connector called the RP-TNC, which stands for Reverse-Polarity Threaded Neill-Concelman, named for its inventor. Another type that Cisco uses is the N connec- tor, invented in the 1940s by Paul Neill at Bell Labs. Different connecters are required be- cause of government regulations. The vendor has to ensure that you use the right antenna with the right product. This doesn’t mean that people can’t make an antenna, but by using vendor-designed antennas, you can be sure that you are within government guidelines for EIRP. Other vendors use connectors such as the Subminiature version A (SMA) and its variants, the RP-SMA and SMA-RS. You also find MC and MMCX connectors on PCMCIA cards. There are a number of others; these are only a few. The important thing, though, is that both sides need to match the type. If the antenna isn’t a direct connect, you need to get a cable from the vendor. When you add a cable between the radio and the antenna, you also add loss. The specific vendor doc- umentation should tell you how much loss. Attenuators If custom cabling is used, you can end up with too much signal, thus causing bleedover into other networks. You can use an attenuator to reduce the signal. You would place an attenuator between the radio and the antenna. Amplifiers If you add a cable between an antenna and its radio, you add loss. To make up for this loss, you add gain. However, you may not be able to add enough gain to compensate. In this scenario, you add an amplifier between the AP and antenna to strengthen the signal. This method is called an active amplifier because it strengthens the antenna. Lightning Arrestors One of the types of antennas discussed in this chapter is a parabolic dish. These antennas offer point-to-point capability between two networks that are far away. This calls for mounting the antenna outdoors, usually on a roof. The antenna cables back to a radio on a bridge or AP and from there makes its way back to the common distribution. In other words, the parabolic dish provides a path back to your wired LAN. So, logically, if a light- ning bolt were to hit the antenna or an access point, it could transfer its energy back along the copper cable. This would result in damage to your entire wired LAN as well. The good news is that you can protect against this by using a lightning arrestor. The Cisco Aironet Lightning Arrestor, illustrated in Figure 5-26, prevents surges from reaching the RF equipment by its shunting effect. Surges are limited to less than 50 volts in about 100 nanoseconds. Because a typical light- ning surge is 2 microseconds, this should provide adequate protection from indirect strikes. Lightning arrestors do not try to stop direct strikes. They also require a ground, as shown in Figure 5-26. 06_1587202115_ch05.qxp 9/29/08 2:45 PM Page 89 90 CCNA Wireless Official Exam Certification Guide Splitters The final topic of this chapter is installing a splitter. Splitters are used mainly in outdoor wireless deployments to split in two a signal coming from a cable and to send it in two di- rections. You could also use it to receive a signal coming from one direction and forward it through another antenna, connected to the same access point, toward another direction. Although this technique can be useful, the drawback is that it greatly reduces the range and throughput by about 50 percent. To Antenna Ground Wire To RF Device Lug Nut Lockwasher Figure 5-26 Lightning Arrestor Key Topi c 06_1587202115_ch05.qxp 9/29/08 2:45 PM Page 90 Chapter 5: Antenna Communications 91 Exam Preparation Tasks Review All the Key Topics Review the most important topics from this chapter, denoted with the Key Topic icon. Table 5-9 lists these key topics and the page number where each one can be found. Complete the Tables and Lists from Memory Print a copy of Appendix B, “Memory Tables,” (found on the CD) or at least the section for this chapter, and complete the tables and lists from memory. Appendix C, “Memory Tables Answer Key,” also on the CD, includes completed tables and lists so that you can check your work. Definition of Key Terms Define the following key terms from this chapter, and check your answers in the Glossary: polarity, diversity vertical polarization, horizontal polarization, circular polarization, om- nidirectional antenna, horizontal (H) plane, azimuth, elevation (E) plane, one-floor concept, Table 5-9 Key Topics for Chapter 5 Key Topic Item Description Page Number Figure 5-5 H-plane and E-plane 76 Figure 5-6 2.14-dBi dipole 76 Figure 5-9 effect of adding gain 77 Figure 5-10 H-plane and E-plane after adding gain 78 Table 5-2 AIR-ANT1728 statistics 76 Table 5-3 AIR-ANT2506 78 Table 5-4 AIR-ANT24120 79 Table 5-5 AIR-ANT2485P-R 81 Figure 5-17 Radiation pattern of a yagi 83 Table 5-6 AIR-ANT1949 yagi 84 Table 5-7 AIR-ANT3338 parabolic dish antenna 86 Table 5-8 AIR-ANT3213 antenna 88 Figure 5-26 Lightning arrestor 90 06_1587202115_ch05.qxp 9/29/08 2:45 PM Page 91 . Pattern –35 –30 –25 –20 –15 –10 –5 0 30330 60300 90270 120 240 150210 180 Figure 5-24 AIR-ANT3213 Left Antenna Radiation Pattern 06_1587202115_ch05.qxp 9/29/08 2:45 PM Page 87 88 CCNA Wireless Official Exam Certification Guide Table 5-8. 2:45 PM Page 89 90 CCNA Wireless Official Exam Certification Guide Splitters The final topic of this chapter is installing a splitter. Splitters are used mainly in outdoor wireless deployments. details of the Cisco AIR-ANT3338. 06_1587202115_ch05.qxp 9/29/08 2:45 PM Page 85 86 CCNA Wireless Official Exam Certification Guide Dual-Patch “Omnidirectional” 5.2 dBi, Pillar Mount Another special type