An accurate antenna factor (AF) is highly desirable for radiated emission and/or normalized site attenuation measurements. An AF is often measured in the presence of a perfectly conducting ground plane and reflectors. For a receiving antenna; the ground plane or reflectors affects the electromagnetic field by altering the magnitude and distribution of the incident field. It also introduces mutual coupling between the antenna and its image beneath the ground. These effects can cause a different measured AF when the same antenna is polarized horizontally or vertically in respect with ground, or calibrated at a different height. Using a monopole antenna: we conduct a numerical analysis to quantify the influence of the ground and reflector on the AF.