When the transmit antenna is different than the receive antenna (always the case when the transmit antenna is an EUT) the far field must be considered to be the greater of the required distance from either the transmit or receive antennas. In the case of an EUT much smaller than the receive antenna, the far field criteria depends on the receive antenna used. This is based not on the over-used lambda/2 pi separation criteria but rather the idea that the far field is achieved when the antenna looks electrically small, the phase difference between a point of observation and the extremities of the antenna are negligible. If "negligible" is defined as a phase difference of lambda/16, then the far field separation becomes 2D^2/ lambda, where D is the maximum linear dimension of the antenna aperture. For a tuned dipole, D = lambda/2, and the relationship simplifies to the far field = lambda/2. Note this is pi times the often cited separation.
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