How Can I Determine Orientation of Antennas?
A good signal coverage can be obtained if you place antennas with a correct polarization direction. From the following figure, you can see that the gain of an AP's omnidirectional antenna is the highest in the direction vertical to the antenna.
Signal radiation of an omnidirectional antenna
When an AP is placed horizontally or wall-mounted, the major lobes of antennas should be placed vertically to receive the optimal signal coverage and then it will have the best connection.