A dual band 2×2 monopole antenna array is proposed. The antenna array elements are fed differentially to align the radiated electric fields so that vectorial addition can be maximized to yield a high gain. Differential feeding is implemented by a 180-degree phase inverter placed in the feeding network. To avoid backside radiation, a metal reflector is placed beneath the ground plane. The proposed antenna operates at 2.75 GHz and 4.7 GHz, yielding a peak gain of 10 dBi and 6.51 dBi respectively. The proposed antenna array exhibits a broadside radiation patterns at the operating frequencies.Index Terms-monopole antenna, high gain, antenna array, phase inverter, planar antenna.

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