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Antenna Types
Directional Antennas
Directional Antennas are designed for use on point-to-point links, or as client antenna in point-to-multipoint applications. Usually, they have the narrowest possible beamwidth and significantly higher gain than other antenna types. Essential rule to be applied is the higher the gain, the lower the beamwidth. Directional Antennas are usually constructed in form of grid or parabolic dish antennas. 
Omnidirectional Antennas
Sector Antennas are designed for use as multipoint side of point-to-multipoint applications. Beamwidth is usually in range of 45° – 180°. Sector Antennas are best choice when cover of certain area is required. From principle of their function, they receive noise only from direction they operate, therefore Sector Antennas are often considered as "better" in comparison to omnidirectional antenna in terms of resistance against noise and interferrence.
Omnidirectional Antennas
Omnidirectional Antennas are designed for use as multipoint side of point-to-multipoint applications. They are best choice to cover of large area with signal. Omnidirectional Antennas distribute radiofrequency signal in all directions - their beamwidth is 360°. Their biggest disadvantage is they receive interferrence from all directions, even from those directions where no client is present. Essential rule to be applied is the higher the gain, the lower the beamwidth in opposite plane. Example: Installation of high gain Omnidirectional Antenna /12 to 15dBi/ on the roof of tall building may lead to the fact, that client trying to get connected on the street level close to this building may experience serious issues with low signal level. This may happen even at low distances from omnidirectional antenna. Another fact to be considered is higher probability of "hidden node" problem when using Omnidirectional Antenna on multipoint side and clients connected in different distances from multipoint. 
Signal polarization
The latest technology intorduced dual polarization in wifi. Antennas are transmitting two signals using two linear polarization planes crossed in 90 degrees. Practical implementations include Point to Point links using N-stream technology from MikroTik and connections using 802.11n standard in 2T2R configuration.
Antenna Gain
Less is more – this sometimes works in building of wireless links as well. The most important factor when building the wireless link is signal-to-noise ratio. It is recommended to choose the antenna with optimal gain rather then maximum available gain. Operating the antenna with useless high gain will cause interferrence with other wireless providers in the area, and will also cause interferrence with ourselves! When using antenna with higher than optimal gain, we receive signal which we would have not even recognized when using optimal gain antenna, and in such case we would noticed much less interferrence. We are also interferred by our own signal reflected from buildings far away from the end of wireless link – that would be much easier situation when using optimal gain antenna.
Antenna Installation
For best possible performance of wireless link, the signal-to-noise ratio is key essential. By installig the antenna on right place, you can improve signal-to-noise ratio, and vice versa, by wrong placement of antenna, you can decrease the ratio. With the change of place where your antenna is installed by even small distance, performance of your wireless link can significantly increase.
Picture A shows antenna placed on the top of the building. Once the antenna is directional or sector antenna, transmitting the signal into certain direction, it is probably not the best place for it.
Picture B shows antenna placed on the different part of the building, where the main part of possible interferrence is shielded by the construction of the building. Antenna placed according Picture B will notice less interferrence from surrounding noise, which will lead to better performance of wireless link. 
