In this paper, the gain variation of a 60 GHz aperture-coupled patch antenna that is dependent on the position on the ground plane is investigated. Despite using the same antenna, the antenna gain is varied as a function of the antenna position on the ground plane with a constant size. The period of gain variation is approximately 0.9λ and the amplitude is fixed with approximately 5 dB. Twenty fabricated test vehicles were measured and compared with simulation results using HFSS software; the measurement and simulation results show adequate agreement. The printed circuit board (PCB) thickness and dielectric constant strongly affect the period of the gain variation.