Cisco Aironet 1700 Deployment Manual page 71

Figure 92 Example of a Poor Installation – Access Point is too High and Buried in Conductive Foil
Remember, the best place for an AP is as close to the users as possible. Avoid metal or conductive objects in the near field (they
cause the radio waves to become directional and increases nulls (dead spots)). If you must mount the AP in a high ceiling, look
at directional antennas to direct (angle down) the signal to the intended target area and always mount dipoles in the correct
orientation.
Figure 93 When Using Dipole Antennas Observe the Correct Orientation (Vertical Polarity)
Cisco Aironet Series 1700/2700/3700 Access Point Deployment Guide
Cisco Aironet Series 3700 Access Point