The configuration of antennas at an Open Area Test site (OATS) for calibrations results in constructive and destructive electric fields. Regardless of the calibration method used, the antenna to be calibrated should be loacted where the field varies slowly. We derive the conditions for a slowly varying field near the location of constructive interference as a function of the height of the transmitting antenna and the field geometry. We show experimental data taken by keeping the receiving antenna fixed per the ANSI standard and scanning the transmitting antenna in height. Repeatability of antenna factor data is improved to less than 1 0.3 dB by avoiding nulls in the field.