Antenna AntennaDirection of Propagation Direction of Propagation Direction of Propagation Direction of Propagation Electric Field Electric Field Horizontal Polarization Vertical Polariza
Trang 1Antenna Antenna
Direction of Propagation
Direction of Propagation
Direction of Propagation Direction of Propagation
Electric Field Electric Field
Horizontal Polarization Vertical Polarization
Figure 5-1 Vertical and Horizontal Polarization
Antenna
Electric Field
Direction of Propagation
Figure 5-2 Circular Polarization
Here is how it works: The two antennas are placed one wavelength apart When the AP hears a preamble of a frame, it switches between the two antennas and uses an algorithm
to determine which antenna has the better signal After an antenna is chosen, it is used for the rest of that frame You can switch antennas and listen to the preamble because it has
no real data As soon as the real data gets there, it uses only one of the antennas
Trang 2Most of the time this happens with a single radio in the AP and two antennas connected
to it This is important because the two antennas cover the same area You wouldn’t try to cover two different areas with the same radio Additionally, the antennas need to be the same If you used a weaker antenna on one side versus the other, the coverage area would not be the same
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 fo-cus, 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 sec-ond 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 omnidi-rectional 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
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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-dBidipole, or rubber duck, shown in Figure 5-6, is most often seen indoors
be-cause 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 termdipole may
be new to you The dipole antenna was developed by Heinrich Rudolph Hertz and is con-sidered the simplest type of antenna Dipoles have a doughnut-shapedradiation pattern.
Many times, an antenna is compared to an isotropic radiator Anisotropic 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
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Figure 5-4 E-plane
AIR-ANT1728 The AIR-ANT1728, shown in Figure 5-7, is a ceiling-mounted omnidirectional antenna op-erating 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 short-ens, as shown in Figure 5-10
Table 5-2 details the statistics of the AIR-ANT1728
Trang 5H-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
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
Key
Topic
Trang 6Key Topic
Figure 5-7 AIR-ANT1728
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60 300
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120 240
150 210
180
Figure 5-8 H-plane of the AIR-ANT1728
Squeeze this Way
Squeeze this Way
Figure 5-9 Effect of Adding Gain
Key Topic
Trang 7Table 5-3 AIR-ANT2506
P
Poollaarriizzaattiioonn Vertical H
E
A
Anntteennnnaa ccoonneeccttoorr ttyyppee Mast-mount indoor/outdoor
Mounting
*This connector type is covered later, in the section “Antenna Connectors and Hardware.”
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
Wider after adding gain!
Squeeze this Way
Squeeze this Way
Figure 5-10 H-plane and E-plane After Gain Is Added
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
Key
Topic
Trang 8Directional Antennas
Directional antennas are usually mounted on walls and have their radiation patterns
fo-cused 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 in-door environment, this kind of antenna usually is placed on walls and pillars In an out-door 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 radi-ation 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
Table 5-4 AIR-ANT24120
G
P
Poollaarriizzaattiioonn Linear Vertical H
E
A
Anntteennnnaa ccoonneeccttoorr ttyyppee RP-TNC M
*This connector type is covered later, in the section “Antenna Connectors and Hardware.”
Figure 5-11 AIR-ANT2506
Key Topic
Trang 9Figure 5-12 AIR-ANT24120
Figure 5-13 AIR-ANT2485P-R Wall-Mounted Patch Antenna
Figure 5-14 Radiation Pattern of the AIR-ANT2485P-R
Trang 10the 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
Table 5-5 provides the details of the AIR-ANT2485P-R
Table 5-5 AIR-ANT2485P-R
G
P
Poollaarriizzaattiioonn Vertical H
E
A
Anntteennnnaa ccoonneeccttoorr ttyyppee RP-TNC M
*This connector type is covered later, in the section “Antenna Connectors and Hardware.”
Key Topic
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Figure 5-15 H-plane of the AIR-ANT-2485P-R