Near-field pattern of a scanning aperture microwave probe: experimental determination and computer simulations

We study imaging properties of a microwave scanning probe consisting of a thin slit aperture in the waveguide endwall. We perform vector measurements of the near-field reflectivity of test samples (conducting dot, stripe, half-plane, plane) at various probe-sample separations and orientations. The experimental results for small objects agree fairly well with analytical calculations and computer simulations and may be described by a quasistatic model. Experimental results for extended conducting objects show strong deviations from a quasistatic model probably due to excitation of the surface waves. Experimental results show several peculiar near-field features which has 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, (iii) excitation of surface waves above conducting surfaces.