8.2.Antennas

• Design goals to keep in mind when selecting an antenna include the following:

  1. Required distance between an AP and a wireless client.

  2. Pattern of coverage area.

     • For example, the coverage area might radiate out in all directions, forming a spherical coverage area around an antenna, or an antenna might provide increased coverage in only one or two directions.

  3. Indoor or outdoor environment.

  4. Avoiding interference with other APs.

• The strength of the electromagnetic waves being radiated from an antenna is referred to as gain , which involves a measurement of both direction and efficiency of a transmission.

• Gain is commonly measured using the dBi unit of measure. In this unit of measure, the dB stands for decibels and the i stands for isotropic .

• A decibel, in this context, is a ratio of radiated power to a reference value.

• The most common formula used for antenna gain is the following:

  GdBi = 10 * log 10 ( G )

Two broad categories of antennas

1.Omnidirectional

• An omnidirectional antenna radiates power at relatively equal power levels in all directions.

• Omnidirectional antennas are popular in residential WLANs and small office/home office (SOHO) locations.

2.Unidirectional

• Unidirectional antennas can focus their power in a specific direction, thus avoiding potential interference with other wireless devices and perhaps reaching greater distances than those possible with omnidirectional antennas.

• One application for unidirectional antennas is interconnecting two nearby buildings.