Understanding External Antenna Deployments
Figure 48 In areas where high amounts of metal is present, a site survey is required
When using 802.11n rates in areas with high metal such as distribution areas or airport hangars, sometimes lower gain antennas (on the ceiling) can perform better as lower gain antennas tend to radiate the signal in all directions increasing the chance that multi-path will enhance the signal. Of course if you have a clear shot, a patch antenna at the end of an aisle at roughly the same height or just above the (WLAN client) is preferred.
Cisco Aironet Series 1600/2600/3600 Access Point Deployment Guide, Release 7.5
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