Common Antenna Types
The two main types of antennas are directional and omnidirectional. In this section you will learn the difference between the two types and look at some of the antennas that Cisco offers. Both send the same amount of energy; the difference is in how the beam is focused. To understand this, imagine that you have a flashlight. By twisting the head of the light, you can make the beam focus in a specific area. When the beam has a wider focus, it doesn't appear to be as bright. While you twist the head of the light, you never change its output. The batteries are the same. The power is the same. The light is the same. You simply focus it in different ways. The same goes for wireless antennas. When you look at a directional antenna, it appears to be a stronger signal in one direction, but it's still emitting the same amount of energy. To increase power in a particular direction, you add gain.
The angles of coverage are fixed with each antenna. When you buy high-gain antennas, it is usually to focus a beam.
Omnidirectional Antennas
There are two ways to determine the coverage area of an antenna. The first is to place the AP in a location and walk around with a client recording the signal-to-noise ratio (SNR) and Received Signal Strength Indicator (RSSI). This could take a really long time. The second method is a little easier. In fact, the manufacturer does it for you. Figures 5-3 and 5-4 show different views of the wireless signal. Figure 5-3 shows how the wireless signal might propagate if you were standing above it and looking down on the antenna.
Note: We say "might" because these values are different for each type of antenna.
This is called the horizontal plane (H-plane) or azimuth. When you look at an omnidirectional antenna from the top (H-plane), you should see that it propagates evenly in a 360-degree pattern.
The vertical pattern does not propagate evenly, though. Figure 5-4 shows the elevation plane (E-plane). This is how the signal might propagate in a vertical pattern, or from top to bottom. As you can see, it's not a perfect 360 degrees. This is actually by design. It's what is known as the "one floor" concept. The idea is that the signal propagates wider from side to side than it does from top to bottom so that it can offer coverage to the floor it is placed on rather than to the floor above or below the AP.
Figure 5-3 H-plane
Figure 5-3 H-plane
Another way to look at this is to imagine an AP, as shown in Figure 5-5. If you draw in the H-plane and E-plane, you can relate the signal to each plane.
Now that you have a better understanding of how to determine the propagation patterns of an antenna, let's look at some antennas.
2.2-dBi Dipole
The 2.2-dBi dipole, or rubber duck, shown in Figure 5-6, is most often seen indoors because it is a very weak antenna. In fact, it's actually designed for a client or AP that doesn't cover a large area. Its radiation pattern resembles a doughnut, because vertically it doesn't propagate much. Instead, it's designed to propagate on the H-plane. The term dipole may be new to you. The dipole antenna was developed by Heinrich Rudolph Hertz and is considered the simplest type of antenna. Dipoles have a doughnut-shaped radiation pattern. Many times, an antenna is compared to an isotropic radiator. An isotropic radiator assumes that the signal is propagated evenly in all directions. This would be a perfect 360-degree sphere in all directions, on the H and E planes. The 2.2-dBi dipole antenna doesn't work this way; rather, it has a doughnut shape.
- Figure 5-4 E-plane
AIR-ANT1728
The AIR-ANT1728, shown in Figure 5-7, is a ceiling-mounted omnidirectional antenna operating at 5.2 dBi.
You would use this when a 2.14-dBi dipole doesn't provide adequate coverage for an area. This antenna has more gain, thus increasing the H-plane, as shown in Figure 5-8.
The easiest way to express the effect of adding gain—in this case, 5.2 dBi versus 2.2 dBi— is to imagine squeezing a balloon from the top and the bottom, as shown in Figure 5-9.
The squeezing represents the addition of gain. The H-plane widens and the E-plane shortens, as shown in Figure 5-10.
Table 5-2 details the statistics of the AIR-ANT1728.
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H-Plane |
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\ \ \ îO |
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r- ~ - _ |
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\ \ I s > \ |
/ / ' / / ' |
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v y |
/A y /J |
x^ i \\ """" J |
/ / / / /// |
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II |
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*This figure is based on an original image from the Wikipedia entry: http://en.wikipedia.org/wiki/E-plane_and_H-plane
*This figure is based on an original image from the Wikipedia entry: http://en.wikipedia.org/wiki/E-plane_and_H-plane
Figure 5-5 H-plane and E-plane
- Figure 5-6 2.14-dBi Dipole Antenna (Rubber Duck) ■■■ Table 5-2 AIR-ANT1728 Statistics
|
Gain |
5.2 dBi |
|
Polarization |
5.2 dBi |
|
H-plane |
Vertical |
|
E-plane |
Omnidirectional 360 degrees |
|
Antenna connector type |
RP-TNC |
|
Mounting |
Drop ceiling cross-member indoor only |
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*This connector type is covered later, in the section "Antenna Connectors and Hardware." |
|
- Figure 5-7 AIR-ANT1728
Key Topic
Figure 5-8 H-plane of the AIR-ANT1728 Squeeze this Way
Figure 5-8 H-plane of the AIR-ANT1728 Squeeze this Way
Key Topic
^Squeeze this Way
Figure 5-9 Effect of Adding Gain
Figure 5-10 H-plane and E-plane After Gain Is Added AIR-ANT2506
The AIR-ANT2506, shown in Figure 5-11, is a mast-mount indoor/outdoor antenna that you mount on a round mast. It is a 5.2-dBi antenna and is omnidirectional.
Figure 5-10 H-plane and E-plane After Gain Is Added AIR-ANT2506
The AIR-ANT2506, shown in Figure 5-11, is a mast-mount indoor/outdoor antenna that you mount on a round mast. It is a 5.2-dBi antenna and is omnidirectional.
Table 5-3 gives details on the antenna.
|
Gain |
5.2 dBi |
|
Polarization |
Vertical |
|
H-plane |
Omnidirectional 360 degrees |
|
E-plane |
RP-TNC |
|
Antenna connector type |
Mast-mount indoor/outdoor |
|
Mounting |
|
*This connector type is covered later, in the section "Antenna Connectors and Hardware."
*This connector type is covered later, in the section "Antenna Connectors and Hardware."
AIR-ANT24120
The AIR-ANT24120, shown in Figure 5-12, is an omnidirectional antenna that is designed to offer higher gain at 12 dBi. Like the 2506, it is a mast-mount antenna.
Table 5-4 provides more details on the AIR-ANT24120.
|
Gain |
12 dBi |
|
Polarization |
Linear Vertical |
|
H-plane |
Omnidirectional 360 degrees |
|
E-plane |
7 degrees |
|
Antenna connector type |
RP-TNC |
|
Mounting |
Mast-mount |
|
*This connector type is covered later, in the section "Antenna Connectors and Hardware." |
|
- Figure 5-11 AIR-ANT2506
Directional Antennas
Directional antennas are usually mounted on walls and have their radiation patterns focused in a certain direction. This is similar to the earlier example of a flashlight (see the section "Common Antenna Types"). The goal is to provide coverage for areas such as long hallways, a warehouse, or anywhere you need a more directed signal. When used in an indoor environment, this kind of antenna usually is placed on walls and pillars. In an outdoor environment it can be seen on rooftops in the form of a parabolic dish.
This kind of antenna provides more gain than an omnidirectional, but again, the shape or radiation pattern is focused. They employ the "one floor" logic discussed earlier (see the section "Omnidirectional Antennas"). This means that they do not have much of a range vertically.
8.5-dBi Patch, Wall Mount
The 8.5-dBi patch is a wall-mounted directional antenna that provides more gain than a basic omnidirectional rubber duck. This results in 8.5 dBi for directional instead of 2.14 omnidirectional. Figure 5-13 shows the Cisco AIR-ANT2485P-R 8.5-dBi wall-mounted patch antenna.
Notice that this is a flat antenna. It is designed to radiate directionally, as illustrated in Figure 5-14. You place this antenna on a wall. By its form factor, it is very discreet.
Figures 5-15 and 5-16 show the H-plane and E-plane. Notice that the radiation pattern is not 360 degrees, even on the H-plane. However, a bit of signal is seen behind the antenna. This is normal and usually is absorbed by the wall that the antenna is mounted to. When
- Figure 5-12 AIR-ANT24120
- Figure 5-13 AIR-ANT2485P-R Wall-Mounted Patch Antenna
- Figure 5-14 Radiation Pattern of the AIR-ANT2485P-R
- Figure 5-15 H-plane of the AIR-ANT-2485P-R
the antenna is mounted above a doorway, the back signal lets a client get the signal from the antenna just as he or she gets to the doorway.
|
Gain |
8.5 dBi |
|
Polarization |
Vertical |
|
H-plane |
66 degrees |
|
E-plane |
56 degrees |
|
Antenna connector type |
RP-TNC |
|
Mounting |
Wall mount |
|
*This connector type is covered later, in the section "Antenna Connectors and Hardware." |
|
Figure 5-16 E-plane of the AIR-ANT-2485P-R 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 warehouse or convention center.
Figure 5-16 E-plane of the AIR-ANT-2485P-R 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 warehouse 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 antenna. 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.
- Figure 5-18 AIR-ANT2410Y-R
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.
Figure 5-19 AIR-ANT1949 Yagi
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 |
Vertical |
|
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." |
|
- Figure 5-20 H-plane of the AIR-ANT1949 Yagi
- 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 enclosed 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 indicate 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 powerful 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 antennas allow polarity to be changed. This is important, because they can be mounted at different angles, and polarity changes how the RF is propagated. Table 5-7 shows the details of the Cisco AIR-ANT3338.
- Figure 5-22 Parabolic Dish Antenna ■•• Table 5-7 AIR-ANT3338 Parabolic Dish Antenna
|
Power |
5 Watts |
|
Gain |
21 dBi |
|
Polarization |
Vertical |
|
H-plane |
12 degrees |
|
E-plane |
12 degrees |
|
Antenna connector type |
RP-TNC |
|
Mounting |
Mast mount |
|
*This connector type is covered later, in the section "Antenna Connectors and Hardware." |
|
Dual-Patch "Omnidirectional" 5.2 dBi, Pillar Mount
Another special type of antenna to consider is the dual-patch 5.2-dBi pillar-mount omnidirectional, 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.
- Figure 5-23 AIR-ANT3213 Dual-Patch 5.2-dBi Pillar-Mount Omnidirectional
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.
-Horizontal (H) Plane Pattern
----Elevation (E) Plane Pattern
-Horizontal (H) Plane Pattern
----Elevation (E) Plane Pattern
Figure 5-24 AIR-ANT3213 Left Antenna Radiation Pattern
-Horizontal (H) Plane Pattern
----Elevation (E) Plane Pattern
-Horizontal (H) Plane Pattern
----Elevation (E) Plane Pattern
Figure 5-25 AIR-ANT3213 Right Antenna Radiation Pattern
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.
|
Frequency range |
2.4 to 2.83 GHz |
|
Gain |
5.2 dBi |
|
Polarization |
Vertical |
|
H-plane |
Omnidirectional |
|
E-plane |
25 degrees |
|
Antenna connector type |
RP-TNC |
|
*This connector type is covered in the next section. |
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