How do you tune that antenna? Just a random piece of wire is certainly not tuned to 2.4 GHz frequency! 3 cm piece of wire is one quarter of the wavelength. If you add something to that is should be 3 cm + n*6 cm in total (straight wire) to keep the antenna in resonance. This would improve slightly the horizontal gain (if the antenna wire is in exact vertical position). I recommend to study some antenna theory before trying these.