The shift in frequency is due
to the radome effect on the antenna. Enclosing the
antenna in plastic loads the radiating element -
in effect, changes the effective dielectric constant
of the antenna substrate/structure.
are 2 possible ways to overcome this. One would
be to re-tune the antenna to compensate for the
added dielectric. The antenna element will be have
to be made smaller (to shift the center frequency
higher by 0.1GHz). Or, another approach would be
to move the ABS plastic away from the radiating
element. Moving the plastic away by a quarter wavelength
(~30mm @2.45GHz) would be a good starting point.
If size is a constraint, you can reduce that spacing
till you get to a point where the frequency shift
is negligible/acceptable/tolerable as the case may