Antenna Specifics
Upgraded antennas can improve WLAN performance, reach and security. 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 38 degree vertical beam and indoor range up to 497 feet.
Figure 1
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 8.5 dBi Patch has a 60H/60V beam with indoor range up to 700 feet. HyperGain's 14 dBi Patch has a narrower 30H/30V beam.
Figure 2
Yagis are higher-gain directional antennas. These cylinders contain a boom supporting thin vertical rods. Signal 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 dBi Yagi (30H/30V) and Cisco's 13.5 dBi Yagi (30H/25V, outdoor range up to 6.5 miles.)
Figure 3
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 25 miles.)
Figure 4
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.
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.
Understanding wireless antennas - part 1 is available here: http://searchnetworking.techtarget.com/tip/1,289483,sid7_gci884306,00.html