Microwave near-field imaging of conducting objects of a simple geometric shape

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Abstract
We study imaging properties of a 90 GHz microwave scanning probe consisting of a thin slit aperture (width of 10–100 μm) in the waveguide endwall. We perform vector measurements of the near-field reflectivity of test samples (conducting dot, strip, half plane, plane) at various probe–sample separations and orientations. Experimental results for small objects agree fairly well with analytical calculations and computer simulations and can be described by a quasistatic model. Experimental results for extended conducting objects show pronounced deviations from the quasistatic model due to excitation of the surface waves. Our experimental results demonstrate several peculiar features which have not been properly addressed in the context of near-field imaging, namely: (i) strong collimation of the fields away from the probe up to a distance equal to the probe width, (ii) very weak phase dependence on distance in the extreme near field, and (iii) excitation of surface waves above conducting surfaces.

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