Synthetic Beamforming for Localized FSS-Based Sensing

Recently, frequency selective surface (FSS)-based sensors have shown potential for structural health monitoring (SHM). FSS sensors are attractive due to their sensitivity to changes in element geometry, inter-element spacing, substrate properties, and local environment. In addition, these sensors are remotely interrogated and present a planar structure, making them an ideal candidate for SHM. Traditionally, FSS sensors are analyzed assuming a uniform (plane wave) illumination. However, practically speaking, the sensor will be illuminated with a non-uniform excitation. In this way, the resolution of the sensor is related to the excitation beam. Therefore, this paper considers a synthetic beamforming approach as a solution for localized sensing applications. Specifically, the effect of distance between the synthetic aperture and FSS sensor is studied, along with the effect of windowing (used to suppress side lobe levels). Measurements were conducted on an FSS sensor based on a square loop element and backed by a ground plane. They were performed on this sensor before and after the introduction of two small copper strips (oriented orthogonally to one another). The results show that, using a focused beam, the presence of the copper strips can be detected.