It always puzzles me why a certain type of antenna since it has no intelligence would make a difference in a wireless coverage depending on her attributes. It certainly depends on which type of wireless architecture as well as what type of coverage you want to achieve.
Antennas play the same role as your cabling system for your wired network. They come in different size and forms and those attributes with their position at your site will determine what type of wireless coverage you will get.
I did not realize the importance of antennas until I have done few outdoor projects. When you deploy WLAN’s in the office environment with integrated antennas, you do not have the ability to play with antenna position because everything is inside. As with external antennas it becomes more challenging because you really have to understand the electric field emitted by the antenna as well as the electromagnetic field. If the electric field does not propagate well or is bounced all over the place, your intended wireless coverage will not be as expected. On top of this if you have a lot of noise in your environment the reliability of your connections may suffer.
Therefore having an understanding of how the electric field propagate (polarization) will help determine what are the best antennas and the proper orientation in order to get a reliable wireless connection. Recently I have gone from vertical polarization to horizontal polarization on a particular outdoor installation and it has reduced my errors on those wireless links tremendously. If you can not afford the expensive Spectrum analyzer you can always change your frequency selection and antenna orientation to move away from interferences. This is not the ideal solution but it requires time if you can not afford the expensive spectrum analyzer.
Ask an expert to do your antenna installation !