Cisco Aironet 1850 Series Deployment Manual page 52

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 41: When Using Dipole Antennas Observe the Correct Orientation (Vertical Polarity)
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