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Antennas direct Radio Frequency (RF) power into a coverage area. Antennas are available which produce differing coverage patterns. The correct antenna for a site is chosen by determining the antenna that provides the coverage pattern best matched to the site coverage requirements. Knowing your environment can help to determine the right antenna and placement.
There are basically two types of antenna:
- Omni-directional antennas have a 360-degree coverage pattern on a horizontal plane. The coverage pattern is torus-shaped (like a doughnut). These antennas are ideal for square or somewhat square areas.
- Directional antennas concentrate the coverage pattern in one direction. This produces an almost conical-shaped coverage pattern (like a flashlight). The antenna directionality is specified by the angle of the beam width. Typical beam width angles are from 90 degrees (somewhat directional), to as little as 20 degrees (very directional). The directed beam allows for a longer but narrower coverage pattern, which is ideal for elongated areas, corners, and outdoor point-to-point applications.
Gaining coverage range:
The increase in coverage within the RF beam width is called the antenna gain, and is measured in dB (decibels). Antenna gain improves the range of the signal for better communications.
Calculating Range:
To calculate the range increase in an indoor site, each 1 dB increase in antenna gain approximately results in a range increase of 2.5%. For an unobstructed outdoor site, each 1dB increase in gain approximately results in a range increase of 5%. Actual results vary depending on the amount and type of obstructions at the site.
Positioning antennas:
The proper positioning (orientation) of antennas at a site helps ensure the maximum coverage area. Antenna should generally be mounted as high and as clear of obstructions as practically possible. Best performance is attained when both transmitting and receiving antenna are located at the same height and in a direct line of sight of each other. Access Points should be positioned on or close to the ceiling.
For omni-directional antennas mounted below the coverage area (on a table, desk, etc.) point the top of the antenna up. For omni-directional antennas mounted above the coverage area (from a ceiling or wall mounted), point the top of the antenna down. Place omni-directional antennas in the center of the coverage area when possible. When ceiling mounted, try to keep antennas at least two feet from sprinkler heads, metal lighting fixtures, etc.
For directional antennas, point at the direction of the coverage area. All antenna characteristics are the same for both transmit and receive. For outdoor mounting of antennas note that the coax RF cable should be kept as short as possible to minimize RF loss - the length of cable that comes with the antenna is optimized. Use coax cable extensions, if needed; however, placing the Access Point close to the antenna and bringing the Ethernet cable to the Access Point is much preferred.
Site Surveys:
Intel recommends site surveys for environments that contain unusually dense obstructions or require complex coverage areas. The site survey software, which helps you determine characteristics of your coverage area and where best to place Access Points, is included at no additional cost with each PRO/Wireless 2011 PC Card. The site survey manual is available on support.intel.com and is also available on your installation CD.
Should you prefer, a reseller or VAR in your area may be able to assist or perform the site survey for you.
This applies to:
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