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