Accuracy of the gain-transfer method for a standard gain antenna and a test antenna with equal aperture dimensions

When implementing the gain-transfer method to measure the gain of an antenna, it is tempting to assume that there should be no measurement errors due to non-plane wave effects in the test zone fields, given that both the test antenna and the standard gain antenna have equal aperture dimensions. The reasoning behind this is that both antennas are exposed to the same amplitude and phase variations in the test zone fields, and therefore the measurement errors should cancel out. However, in this paper it is shown through simulated gain measurements, that equal aperture dimensions can indeed result in significant measurement errors if the incident wave is not plane.