Upgraded antennas can improve WLAN performance, reach and security. As explained in the first part of this tip, dipole antennas included with 802.11 products radiate signal 360 degrees around the antenna's axis. After-market antennas focus transmission more narrowly, boosting power where you need it by decreasing signal where you don't.
Types of antennas
Like dipoles, Omni antennas radiate signal 360 degrees horizontally. But they increase gain by flattening the signal, producing a vertical beam between 80 degrees (modest gain) and 7 degrees (high gain) – see Figure 1. Recall that gain makes signal travel further. For example, Cisco's 5.2dBi Ceiling Omni has a 40 degree vertical beam and indoor range up to 397 feet.
Figure 1
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Directional antennas focus signal in both planes. Patch antennas are flat directional antennas, mounted flush on walls or ceilings. They produce hemispherical coverage, spreading away from the mount point (Figure 2) at a width of 30 to 180 degrees. Concentrating signal on this smaller area further increases range. For example, Cisco's 9.5 dBi Patch has a 50H/43V beam with indoor range up to 1030 feet. HyperGain's 14 dBi Patch has a narrower 30H/30V beam.
Figure 2
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Yagis are higher-gain directional antennas. These cylinders contain a boom supporting thin vertical rods. Signal...
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propagates off the front of the boom somewhat like blown bubble gum (Figure 3). Note that some signal (back lobes) fall behind the boom. Yagis create higher gain by producing narrower beams (20-80H, 14-64V). Examples include HyperGain's 14.5 dBi Yagi (30H/30V) and Cisco's 13.5 dBi Yagi (30H/25V, outdoor range up to 18 miles.)
Figure 3
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Parabolic dish or grid antennas are concave panels or bowls that produce an extremely narrow beam (4-25 degrees horizontal/vertical), like a rocket with exhaust spreading from the base (Figure 4). Examples include HyperGain's 24 dBi Grid (8H/V) and Cisco's 21dBi Dish (12.4H/V, outdoor range up to 26 miles.)
Figure 4
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Where they can be helpful
Now that we know what they look like, where should we use these antennas?
Standard-equipment Dipoles are ok in densely-populated office floors, including cubicle bullpens where stations are centered around the AP. But dipoles waste (leak) signal when the AP is located near an outside wall or corner or when you need to cover just one floor.
After-market Omnis are better for high-ceiling industrial and retail environments – factory floors, warehouses, or "big box" stores where antennas can be suspended from the ceiling at the center of large open areas. But Omnis are not good in long, narrow workspaces where antennas cannot be centrally located.
Patch antennas are better for covering single-floor small offices, small stores, and other indoor locations where APs cannot be placed centrally. For example, mount patch antennas unobtrusively on the back wall of a store. Back lobes do create some leakage, but far less than an omni-directional antenna would in the same situation.
Yagi antennas are better for corridors, hallways, tunnels, long narrow building, and point-to-point medium range connections between outdoor bridges (for example, connecting two buildings in an office park or campus). Be wary of back lobes, but the Yagi's narrow beam will reduce unwanted peripheral exposure in the focal direction.
Parabolic antennas are better for long-range outdoor point-to-point connections – for example bridges that are miles apart. They require more precise installation to aim signal where you want it, but have the very high gain necessary to reach such distances.
802.11a/b/g access points usually have a pair of diversity antennas. These APs track which antenna receives the fewest errors so that they can send the next transmission through that antenna. Newer draft 802.11n products often have four or even more antennas. However, instead of choosing the single best antenna, those APs transmit data streams through all antennas at once. Receivers then recombine those streams to boost total WLAN bandwidth. In Part 3 of this series, "Understanding 802.11n wireless antennas," we take a closer look at how 802.11n Multiple Input Multiple Output (MIMO) antennas actually work.
Conclusion
I hope these descriptions help you to visualize what after-market antennas can do, and how they might benefit your WLAN. Within each category, you'll find a variety of products with a wide range of beam widths and signal gains, so shop carefully to find the antenna that's best for your workspace. With the right antenna, you don't have to sacrifice performance for security - by focusing signal, you can improve both at once.
Read Understanding wireless antennas -- Part 1 http://searchnetworking.techtarget.com/tip/1,289483,sid7_gci884306,00.html
About the author:
Lisa Phifer is vice president of Core Competence Inc., a consulting firm specializing in network security and management technology. Phifer has been involved in the design, implementation, and evaluation of data communications, internetworking, security, and network management products for nearly 20 years. She teaches about wireless LANs and virtual private networking at industry conferences and has written extensively about network infrastructure and security technologies for numerous publications. She is also a site expert to SearchMobileComputing.com and SearchNetworking.com.
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