Some Radio network interface cards (NICs) and access points have integrated antennas that you can't change. For example, laptops such as Apple iBook integrate the antenna within the cover or body of the device, which is not visible or changeable by the user. Some radio NICs and access points also use permanently mounted antennas. With these types of products, you have no choice but to use the antenna the vendor supplies.
Other wireless LAN devices have antennas that are interchangeable. In fact, it's a good idea to purchase access points with removable antennas. These allow more flexibility by enabling the selection of an antenna having characteristics better suited for your specific application. The more common antenna types for wireless LANs have omni-directional and directional radiation patterns. Omni-directional antennas propagate RF signals in all directions equally on a horizontal plane (i.e., throughout the facility), but limit range on the vertical plane. This radiation pattern resembles that of a very large doughnut with the antenna at the center of the hole.
Omni-directional antennas, having gains ranging up to 6 dB, apply to most applications inside buildings. Omnis provide the widest coverage, making it possible to form somewhat circular overlapping cells from multiple access points located throughout the building. Most access points ship standard omnis having relatively low gain.
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