  Near-field imaging experiments exist both in optics and microwaves with often
different methods and theoretical supports. For millimeter waves or THz waves,
techniques from both fields can be merged to identify materials at the micron
scale on the surface or in near-surface volumes. The principle of such
near-field vector imaging at the frequency of 60 GHz is discussed in detail
here. We develop techniques for extracting vector voltages and methods for
extracting the normalized near-field vector reflection on the sample. In
particular, the subharmonic IQ mixer imbalance, which produced corrupted
outputs either due to amplitude or phase differences, must be taken into
account and compensated for to avoid any systematic errors. We provide a method
to fully characterize these imperfections and to isolate the only contribution
of the near-field interaction between the probe and the sample. The effects of
the mechanical modulation waveform and harmonic rank used for signal
acquisition are also discussed.
