Omnidirectional antenna

An omnidirectional antenna is an antenna which radiates power uniformly in one plane, with the radiated power decreasing with elevation angle above or below the plane, dropping to zero on the antenna's axis. This radiation pattern is often described as "donut shaped". Omnidirectional antennas oriented vertically are widely used for nondirectional antennas on the surface of the Earth because they radiate equally in all horizontal directions, while the power radiated drops off with elevation angle so little radio energy is aimed into the sky or down toward the earth and wasted. Omnidirectional antennas are widely used for radio broadcasting antennas, and in mobile devices that use radio such as cell phones, walkie-talkies, Wifi, cordless phones, GPS as well as for base stations that communicate with mobile radios, such as police and taxi dispatchers and aircraft communications.

Antenna omnidirectional antenna typically has a 360-degree radiation pattern. Usually the polarization field E is vertical. Strengthening the omni antenna is usually very low about 3-12 dBi only. Usually used for the connection Point-To-MultiPoint (P2MP). Good enough for 1-5 km distances, especially if the directional antenna with high reinforcement in use on the client side.

Shown in the picture below is the radiation pattern of omnidirectional antenna 140 RFDG to 2.4GHz made by RF Linx (http://www.rflinx.com). Horizontal radiation pattern close to 360 degrees. Basically horizontal polarization radiation field-E. For comparison, the vertical radiation pattern is very thin pieces.

All this means that only stations that are in a 360-degree radiation will be examined by the omni antenna. This omni antenna can not provide service at the station located above the antenna.

Ref : 

http://en.wikipedia.org/wiki/Omnidirectional_antenna

http://opensource.telkomspeedy.com/wiki/index.php/Omni