TIP :-  When installing an Omni antenna on the rooftop - remember it can be a target for any local lightning activity, a direct strike or energy from a local strike in the area can be absorbed into the omni,cables and mast. 
ENSURE this mast is EARTHED at all times, A cable from the bottom of the metal mast heading down to an earth point such as a rod driven into the ground makes a good lightning protection system. Lightning follows a direct path to earth in most cases, no bends in the earthing cable.  Fit a surge arrestor in line with the cable to protect your Access Point equipment. Remember to earth this surge arrestor as well to your earth system.
Check out http://www.techno.net.au/wireless/earthing for my setup on NodeABJ
Ernie