CCNA Wireless Official Exam Certification Guide part 12 docx

13.5 Yagi Antenna The 13.5 yagi antenna is a directional antenna that offers a very direct radiation pattern.. This yagi is a high-gain antenna at 13.5 dBi... 21-dBi Parabolic Dish The 2

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 anan-tenna

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

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Figure 5-16 E-plane of the AIR-ANT-2485P-R

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 Topic

Table 5-6 AIR-ANT1949 Yagi

*This connector type is covered later, in the section “Antenna Connectors and Hardware.”

Key

Topic

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Figure 5-20 H-plane of the AIR-ANT1949 Yagi

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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 inin-dicated 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

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

P

G

P

H

E

A

Anntteennnnaa ccoonneeccttoorr ttyyppee RP-TNC M

*This connector type is covered later, in the section “Antenna Connectors and Hardware.”

Key

Topic

Figure 5-22 Parabolic Dish Antenna

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

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Figure 5-24 AIR-ANT3213 Left Antenna Radiation Pattern

Table 5-8 AIR-ANT3213 Antenna

Antenna connector type RP-TNC

*This connector type is covered in the next section

Key

Topic

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

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Figure 5-25 AIR-ANT3213 Right Antenna Radiation Pattern

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

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

Topic

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

Figure 5-10 H-plane and E-plane after adding gain 78