The "normal" AP's (like the 305) has built-in downtilit antenna's. These should always be mounted on a ceiling, facing downwards.
You can also use a model with external antenna's (like the 304) it will allow you to install different antenna's, including dual polarized (ANT-28 and ANT-35a) and even an multi polarized (ANT-38)
Please see a complete list of all available antenna's here:
https://www.arubanetworks.com/products/networking/antennas/