Synthetic Aperture Imaging of Near-Field Scatterers Mutually Coupled with an Antenna Array

In this study, we propose a synthetic aperture imaging technique for near-field scatterers mutually coupled with an antenna array, such as supporting structures, fixtures of the antenna, and a radome enclosing the antenna. Mutual coupling is a phenomenon, in which the scattered and reradiated waves from the array elements and the near-field scatterers are received by the array elements, and it is known to cause undesired effects in array signal processing, such as direction-of-arrival estimation. Therefore, we need to design the antenna array with the goal of reducing the unwanted scattered and reradiated waves as much as possible, and we require a method to evaluate the scattering intensity for validation of the antenna design. For this purpose, we utilize the bistatic synthetic aperture radar measurement, whereby we mount the receiving antenna array under test on a turntable for azimuth rotation and irradiate this antenna with a fixed transmitting antenna. The measured signals are synthesized to image an area that contains both the array elements and scatters of interest. Because the spatial separations between the array elements and the scatterers are small, the direct waves from the transmitting antenna to the receiving elements result in serious artifacts in the reconstructed image. To suppress this effect, we apply a direct wave filter as a preprocessing method for image reconstruction. We demonstrate the proposed imaging scheme via data sets of numerical electromagnetic simulations and anechoic chamber measurements.