Antenna Type

Basically we have several types of antennas that can be used in Wireless Metropolitan Area Network (WMAN), namely:

• Omnidirectional Antenna - Antenna is usually used in Access Point (AP). This type antenna has a 360-degree radiation pattern.
• Sectoral antennas are usually used in Access Point (AP). Usually have a higher gain than omnidirectional antenna, but the area is on the cover is usually only about 90-180 degrees.
• Directional Antenna (Antenna Steering) is usually used on the client side. Usually has a very high gain and are directed to the Access Point (AP). The example used in the Wireless Internet, such as satellite dish antennas, Wajanbolic e-goen or antenna cans.

Some countries, including Indonesia, requires that all radio systems in the certification. This means that only the antenna on the test & in the certification along with his radio that can be used. Unfortunately, the application of such rules would be very difficult because it is very easy to change the configuration tool.

Ref : http://opensource.telkomspeedy.com/wiki/index.php/WiFi:_Tipe_Antenna

Sectoral Antenna

Sectoral Antenna sometimes called the Antenna Patch Panel is basically not much different with omni antenna. Typically used for connection to the Access Point Point-to-Multi-Point (P2MP). Generally sectoral antenna has vertical polarization, some of which also has a horizontal polarization.

Sectoral Antenna reinforcement generally have higher than about 10-19 dBi omni antenna. Very good to provide service in areas within a distance of 6-8 km. Strengthening of sectoral antenna height is usually in compensation to the width of a narrow radiation pattern of 45-180 degrees. Clearly, in the service area that can become more narrow, and this is very profitable.

In the figure below show the sectoral antenna radiation pattern. In general, more radiation antenna to the antenna face, not a lot of radiation behind the antenna sector. Radiation vertical pieces do not vary much with omni antenna.

Sectoral Antenna usually put on top of a tall tower, therefore, usually on tilt a bit to provide services to areas underneath.

Ref : 

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

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

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