Csar Imaging of Electromagnetically Coupled Conducting Scatterers

Abstract

Chipless RFID with small, printed metal tags have been proposed as a cost-effective alternative to chip-based technologies. A potentially viable configuration is to image the patches of different shapes, sizes, and orientations within a tag with a tabletop-scale synthetic aperture radar (SAR), operating in the V or W band. Information is encoded into, e.g., polarization, resonance characteristics, and phase of the scattered signal. The effect of electromagnetic coupling and sidelobe interference between closely spaced metal patches on SAR image has not been addressed in prior studies. To be specific, we analyze 60 GHz circular SAR (CSAR) imagery of subwavelength patches separated by distances on the order of wavelength. The scattered field is calculated with the method of moments (MoM) to account for EM interaction. The field is then used to form CSAR image with the polar formatting algorithm (PFA). Significant distortion of the CSAR image is found at this scale. Sidelobe interference causes image distortion and up to 7 dB of intensity modulation with patch separation. EM coupling produces an "interaction image," an artifact that extends between the patches. The source of this effect is traced to induced currents and charges residing on the patches' inner edges. Increasing system bandwidth or changing the incidence angle has minimal effect on both classes of image artifacts, highlighting the importance of accounting for them in practical system design and subsequent information processing.