Most wireless antenna and wireless access point manufactures usually say the higher the better went mounting. This is true but you must take in account: coverage area, height of the building, type of antenna and weather.
Wireless Antenna - First you need to decide what type of antenna you are going to use. If you are going to use a directional antenna then you will be able to tilt it downward if needed to cover you needed area. But if you mount the antenna two high you will have a dead zone between the building or ceiling to the ground level. You will not want to tilt a omni directional antenna downward because you will be sending half of your signal up in the sky.
Another common scenario is to use a Omni directional antenna in conjunction with your directional antenna to avoid dead spots. When choosing a directional antenna make sure it's radiation pattern suits your needs. If you need to give a long hallway wireless coverage then you will need to ensure it's radiation pattern doesn't leave dead spots.
One 8dbi directional antenna could have a signal pattern of 60 degrees horizontal and vertical and a 14dbi could have a 30 degree signal pattern. If you used 14dbi directional antenna the room closest to the wireless access point could end up being dead spots.
Coverage Area - The most important thing is that you can provide a constant secure signal with out it costing too much money. That is really how you decide how to place your access points. Usually the higher the better because the height lets gives more options. Just take in account that you are not leaving dead spots close to the access point / antenna.
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