plastic antenna enclosure
my experience is that you can put antennas inside a plastic enclosure, but you have to design to minimize the effects.
If you can have an air gap between the antenna element itself and the plastic, that helps a lot. You want to make sure that the antenna stays "tuned" to your operating frequency, and adding a plastic dielectric nearby can lower the resonant frequency.
Small ceramic chip antennas, due to their very high dielectric constant, are less prone to detuning from the dielectric of the housing.
If you are using a wire, coil, or helical antenna, and it is fiting into a specific spot in the plastic body, then you want to make sure it goes into the spot THE SAME WAY each and every time. If the metal part of the antenna can randomly flop around inside of the plastic body, you are going to have random tuning effects. You can put a small reactive tuning element to in series with some antennas, and trim that element until the antenna+housing are tuned up. You can design the plastic housing with pins, slots, etc, to capture the metal antenna so it does not move around.
The metal and components on the overall PCB will tend to change the directional pattern of the antenna. LCD screens, with metal bottoms, screw things up too. Instead of a nice omnidirectional pattern, you will end up with an "egg-shaped" pattern, due to the non symmetry of the RF ground plane in a standard RF board layout. Empirically it seems that loop type antennas are less effected by ground plane shape than monopole type antennnas.
Rich