You haven't been specific, but this sounds like an antenna housing for something like a WiFi or ISM band application (which is what I will assume). In these cases, you should be choosing your plastic almost like you would choose any other plastic for a harsh marine enviroment exposed to the elements and UV that is protecting electronics.
The material is only important when enclosing a highly directional antenna, or a high power antenna as found in a marine radar where the antenna and it's pattern are of high importance. For lower power use where you are already working fairly omnidirectional, then only how the dielectric of the plastic affects the antenna tuning is of interest. Your primary interest is still in protecting the antenna. Even some slightly conductive ESD plastics are almost completely transparent to RF, unless they are heavly loaded with carbon or conductive additives.
I once analyzed several antennas sold by big name antenna "distributors" for mobil and fixed applications only to find that the plastics used were of a variety of inexpensive types. The antennas all worked well, but testing a few in weather proved that simple UV additives were not present, the plastics had no impact resistance, and shattered at freezing temperatures with no external stress applied! (due to manfacturing assembly stresses) and they all failed after a few weeks. A subsequent tear-down showed that all must have been built in china, acid core solder was used internally in some!, there was no sealing, coaxes filled with water, One used audio cable, not coax and the internal assembly quality was non-existant.