Axial-null illumination (ANI) is proposed to simplify the calibration of microwave imaging systems. The illumination also enhances the spatial resolution. ANI can be achieved with various array configurations, but a minimum of two transmitting antennas are required, which is a well-known form of differential illumination. Here, ANI is achieved with four transmitting antennas, and its implementation is investigated in a planar scanning scenario. The receiving antenna resides at the radiation null of the ANI array. Back-scattered reception requires an antenna at the center of the ANI array whereas forward-scattered reception requires an antenna aligned with the ANI axis, but on the opposite side of the imaged volume. The most important advantage of the proposed imaging setup is that it eliminates the need for background (or baseline) measurements, thus simplifying the system calibration. Also, it is proven that at least two-fold improvement in the spatial resolution can be achieved in near-field imaging scenarios compared to the conventional single-source illumination.
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