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Near-field millimeter-wave techniques have effectively been used for nondestructive testing (NDT) and imaging applications for over a decade. The interaction of the fields and a structure under test (SUT) in the near field of a probe is more complex than that of the far-field interaction. In the near field, the distance between the probe and the SUT, which is referred to as the standoff distance, is an important measurement parameter, and when optimally chosen, it can significantly improve detection sensitivity. However, undesired changes in this parameter can adversely influence the detection outcome to the extent that a target may be totally masked. Consequently, in the past, several different methods and remedies have been proposed to eliminate or drastically reduce this adverse influence, each with its own limitations. In this paper, a novel method involving a probing waveguide aperture loaded with two small modulated antennas is introduced, which operates in a differential mode and is capable of automatically eliminating undesired changes in the standoff distance during testing. In addition, this differential probe efficiently overcomes the limitations of the previously developed methods. The proposed probe is based on electronic modulation of the dominant aperture field of the rectangular waveguide using p-i-n diode-loaded dipoles symmetrically placed in the aperture. This paper presents the design of this unique probe, and the results show that the adverse effect of the standoff distance variation can be eliminated or otherwise significantly reduced by noncoherently subtracting the signals measured at two different aperture modulation states.